add chatcommand to allow user to switch to mobile friendly controls

[SwissalpS]

Sokomine created a helper tool to set replacer on mobile devices.
I think it is complicated for user and I prefer not to add yet another tool.

Instead abstract the controls check. This allows us to use alternative sets and will help with adding fully customizable controls and param settings via digilines. (card swiper or other devices).

/replacer mobileControls [on|off]

The setting should be saved in player meta instead of the tool itself.

When mobile mode is active:

• right-click (place / short tap) ==> place
• left-click (use / long tap) ==> replace
• sneak (climb down / down arrow) + any click/tap ==> opens formspec
• the formspec has an additional button "read settings next time a node is clicked/tapped"

Using special key doesn't work well on mobile (fast is a 'state' there and needs to be toggled by opening a menu)

Personally I think anybody wanting to do serious building using a mobile device should get a mouse and keyboard. So I'm not giving this feature a lot of importance. What motivates me is the part that prepares for digiline reading/setting paramaters and controls.

Side note: if controls can be set per tool via digilines, then it would make sense to let user customize the title/description of the tool too so the tools can be more easily identified. Possibly even allow user configured colour overlay.

[SwissalpS]

I've tried again on a phone with Minetest 5.8.1 and I didn't find any extra inconvenience.
Yes, special/run is a bit weird but replacer and technic replacer can be used fine without it.
There is a down arrow and an aux1 button allowing player to set replacer to any node and mode.