Added keyboard event handling to tab widget

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added event handling to frame widget

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added keyboard interaction to button widgets

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Implemented drawing of a focus box around focused widgets

 - When user first keyboard-interacts with recently focused widget,
   a focus box is drawn to remind the user where the focus is
 - Widgets have focus policy flag to check whether they accept focus,
   focus box should be drawn and when they lose focus

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added tab-key focus switching

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>