Shortcuts for activating and showing/hiding layers

[AlfonZ]

I'd like to activate, show & hide layers using just keyboard shortcuts.

I have prepared a patch that creates shortcuts for this.
There will be 10 shortcuts for activating and 10 shortcuts for showing/hiding layers.
At first I thought to assign them to CTRL-1 to CTRL-0 and CTRL-SHIFT-1 to CTRL-SHIFT-0, but I wasn't sure if that's the right choice so I left them unassigned and user has to configure them in preferences.
Layers will be index-referenced. Does anybody use more than 10 layers at once?

This is my first patch for JOSM, so it needs proper review, especially shortcut names and texts for translation. Any feedback is welcome.

[AlfonZ]

patch