listbox: Mark activatable rows with a .activatable style class

listbox: Mark activatable rows with a .activatable style class

Do not use .button anymore.

This is for 2 reasons:
1. The styling is seperate in our themes, so it doesn't make sense to
   share the style class.
2. Due to the shared styling of .buton, listbox rows inherit all the
   special case styles that exist for buttons - such as linked buttons,
   header buttons, entry buttons, spinbutton buttons, etc. This means
   that the code has to check all these special cases all the time and
   for listbox rows, this is very slow.