Adds "screen saver" mode to signer app

[italo-sampaio]

The screen saver screen is just a completely black screen with no active pixels. This
PR introduces the logic to switch to this "screen saver" screen after 30 seconds
of inactivity in the signer app. When any of the physical buttons is pressed, the usual
Signer running... screen is presented again.

Also, I took the chance to remove some unused states and their corresponding logic, since they were not being used.

[amendelzon]

Works like a charm!

A few things I think we should try to improve. I'll write them in order from most to least important. Optional items marked with a (*):

• There are no tests. I'd probably separate some of the logic and write some minimal unit tests so that we can catch potential issues in the future.
• There are no comments whatsoever.
• The name ui_idle is misleading, since it's actually the only state in which the screen shows something (and is activated in response to user actions). So maybe rename it to something more meaningful like ui_info?
• (*) Functionality wise: does it make sense resetting the timer if the user keeps pressing the button? There's not that much info that a user can't finish reading it in 30 seconds. What do you think?
• (*) This is out of scope, but if it's not too hard, we might as well seize the momentum: how hard would it be to have the signer version displayed underneath the "Signer running..." message? As in: Version: 5.2.0.

[italo-sampaio]

Works like a charm!

A few things I think we should try to improve. I'll write them in order from most to least important. Optional items marked with a (*):

• There are no tests. I'd probably separate some of the logic and write some minimal unit tests so that we can catch potential issues in the future.
• There are no comments whatsoever.
• The name ui_idle is misleading, since it's actually the only state in which the screen shows something (and is activated in response to user actions). So maybe rename it to something more meaningful like ui_info?

Thanks for the comments, I'll tackle them.

• (*) Functionality wise: does it make sense resetting the timer if the user keeps pressing the button? There's not that much info that a user can't finish reading it in 30 seconds. What do you think?

Good point. This is the usual behavior of a screensaver, that's why I went that way, I think it's more intuitive for the user. I agree there's no point in keeping pressing the buttons since their only side effect is showing the same static screen, but if for some reason you keep pressing, doesn't it look strange if the screen goes off on you even though you just pressed it? IDK, I don't have a strong opinion on that, what do you think?

• (*) This is out of scope, but if it's not too hard, we might as well seize the momentum: how hard would it be to have the signer version displayed underneath the "Signer running..." message? As in: Version: 5.2.0.

Sounds good, will give it a try. Doesn't seem to be hard to implement and surely adds value.

[amendelzon]

Thanks for the comments, I'll tackle them.

Thanks!

Good point. This is the usual behavior of a screensaver, that's why I went that way, I think it's more intuitive for the user. I agree there's no point in keeping pressing the buttons since their only side effect is showing the same static screen, but if for some reason you keep pressing, doesn't it look strange if the screen goes off on you even though you just pressed it? IDK, I don't have a strong opinion on that, what do you think?

Fair enough, that makes sense. Let's leave it as is.

Sounds good, will give it a try. Doesn't seem to be hard to implement and surely adds value.

Great! I wouldn't spend a lot of time working on this, so only if it turns out to be pretty straightforward.

[italo-sampaio]

Ready for review!

Tackled all suggestions except for displaying the current signer version. Although feasible, this is not so simple since our current versioning scheme uses hexadecimal constants for the version, and our screens are declared as static const, meaning we need to know string that is shown in that screen at compile time.

To add this feature we would need to either modify the version definition to decimal values or change how our screens are declared (or even add fancy macros to do the conversion). Since this is out of scope and we agreed not to spend too much time on this suggestion, I left this for another time. In any case, let me know if you think it's worth proceeding with it anyways.

[amendelzon]

Thanks for tackling all these comments! Made a small comment regarding the unit tests, let's see what we decide on that before moving forwards.

[amendelzon]

LGTM!

Thanks for the changes!