Modernize ActionButton code

ActionButton is now an AbstractButton subclass. This allows it to inherit API, interaction and accessibility related behaviors that most buttons should have.

LogoutButton was also changed since it's an ActionButton subclass. labelRendering was removed because the linked bug can no longer be reproduced. circleVisiblity and circleOpacity were removed because they don't seem to make the difference the comment says they make.

Basically a purely technical replacement for !4010 (closed)

We can discuss making these use normal PC3 toolbutton graphics in the linked MR or somewhere else, but this is purely for code modernization.