Add "keyboard" touch button to the touch overlay

[leonstyhre]

Hi!

As discussed separately via email, it would be great to be able to have a "keyboard" button directly on the touch overlay.
For MESS computer systems you frequently need to open the keyboard to press things like space or return or 1 or 2 for player selection etc.

Today it's necessary to go to Options and and then select Keyboard which is a bit of an extra step.

Thanks!