The code to compare buttons from the previous to the current frame, to
see if they match (an thus should keep the same state) was quite
generic, and didn't allow much flexibility/customization. For some
cases this isn't enough, and a more specific comparison is needed. Say
if some buttons don't actually store comparable data themselves, only
via the button context. This was the case in D14653.
It was possible to set context pointers for buttons via the layout, but
not for buttons in places where the layout system wasn't used (where
buttons are placed manually). This is needed for buttons in the
Outliner, see D14653.
