Improve key handling for welcome screen

This makes the Projects/Examples/Tutorials buttons work
by keys in addition to mouse.

Instead of only respecting the mouse for button presses
use an ExclusiveGroup so that pressing space also changes
the mode (before pressing space on the Examples button
would change to example mode but not highlight the button).

Change-Id: Ib4704fc1bb09767b6b69e2a6364ceca4525a0676
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reference only public properties

This doesn't change anything, but the control property
is "private" to button, prefer using the public root
"button".

Change-Id: I2dc4e92367ff196e2d02b74d6f97ebc0ee355a76
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Welcome Screen accessibility improvements

Change-Id: I2fa2f98566378a5f9ad12e3c74cbe293d97eb2a4
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>