Add MenuBar to MML's startup GUI

[SJuliez]

Adds MML's menubar to the startup window, obviously with some menu items disabled. This adds a few missing abilities to the startup like loading from file.

Removes most accelerators as they seem to a) not work or b) not make too much sense for menu items that don't need acceleration.

Also removes a few menu items that seem no longer necessary like getting the unit summary from cache, as the cache is always available by using load unit (Ctrl-U) which is probably more present to the user than the menu item in question and the unit summary is shown in the selector itself.

[HammerGS]

Thank you! Love this the ability to load a unit from file is something I've missed/wanted off the splash for a while.

I noticed how outdated the Help-Record Sheet Images text is.

As part of this could we swamp the text in menu.properties line 89 to this
menu.help.imageHelp.text=To add a fluff image to a record sheet. Please see the following link\n
https://github.com/MegaMek/megamek/wiki/How-do-I-get-Fluff-Images-to-work

I'd do up change to Master, but don't want to create merge conflict of you. :)

[SJuliez]

Sure, I'll add it!

[neoancient]

Merge conflict anyway :(

[neoancient]

Even though it doesn't actually provide the requested ability to disable the splash screen, I would say that the addition of the menu provides the functionality that was previously not available at startup, and should close #411.

[SJuliez]

I have a change in another branch coming that'll add a config for starting with the most recent unit or with a new unit instead of the splash main menu :)

[SJuliez]

Solved!