Button group adjustments

In order to support togglable button groups two changes are made:
* Avoid buttons in the group to have two borders (improvement only
  for modern browsers).
* Allow the active state to be settable using a class (mw-ui-checked)
  with the same style as :active but not depending on hover.

Change-Id: I6152c963f15420f1dcb3086e077163d28d6c6e2d
Merge "Button group adjustments"