Merged and simplified button codepaths

 - Button only generat BUTTON_IN and BUTTON_OUT events
 - button can be created as "toggle button" that behaves the
   same way a checkbox does
 - If a radio button is not connected to a group, it behaves the
   same way a checkbox does
 - Any type of button can be connected to a group that will then
   behave like a radio button menu

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

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