Use GLib.Action for buttons

[danirabbit]

Use GLib.Action with action targets for buttons.

This avoids conflating the button label with its function and uses the built-in mechanism for activating actions in this way instead of adding a property to a special button subclass

these are probably dumb action names, open to suggestions

[jeremypw]

Looks like a useful cleanup. Haven't tested it yet but a couple of thoughts:

• Is it worth making any of the non-mathematical button labels translatable?
• Could take the opportunity to rename the action target in the handler to something other than label as it is not a label. Maybe token?
• action_button might be better named action_insert as that is what it does.

[danirabbit]

@ryonakano any thoughts on which button labels should be translatable that currently aren't?