gui: keyboard: support longpress label offset, code cleanup

To position the longpress label:
<longpress font="..." textcolor="..." x="5" y="0"/>
x and y is the distance from the upper right corner.

- extract method GUIKeyboard::DrawKey
- clean up handling of currently pressed key
- integrate key highlighting into main key drawing loops

Change-Id: I80ccf2975d30cffa2a48627a74ab3693c84e4fa4