Add "clicked" effect to all buttons as feedback

Add feedback to all buttons in TWRP. It is very annoying not to know if one just missed the button, or if it is doing something, especially in the "Mount" section and when rebooting, as it does umounting, which can take a while.
It looks pretty much like the highlight in keyboard (I kinda copy-pasted from that). Perhaps the color could be moved into xml files, but I don't think that it is necessary, because it is just half-transparent "something" color.

PS: you should really decide if you wanna to indent with tabs or with spaces.