In the FindReplaceOverlay, the assignment of actions to buttons and
checkboxes as well as their shortcuts, and the correlation of shortcuts
with buttons is not supported by proper OO design by needs to be done
manually. This leads to higher code complexity in the FindReplaceOverlay
and makes it prone to error when doing changes in that class.

This change extracts the actions assigned to buttons into a separate
class that combines these actions with associated shortcuts. They are
then automatically registered for the buttons they are assigned to.