frontend: Highlight control dock buttons using style class

Checkable buttons are troublesome because QAccessible interprets them as
checkboxes, and doesn't pass the "clicked" signal to them (see
QTBUG-110737). Instead, we only get the "toggled" signal, which also
gets triggered by things other than user input (i.e., the button getting
changed by the program).

Making them uncheckable means that they behave like normal buttons
again. To style them, we can just add a class and address them that way.
Note that uncheckable here means *actually* not checkable - unlike the
cursed NonCheckableButton class previously used that makes a button
that *is* checkable ignore the user input (and only allow check state
changes via code, effectively making it solely a styling tool).