This is a Gtk.Button that has been customized to act similarily to a
Gtk.ToggleButton, but instead of giving the button a "pressed in" look
it changes the opacity of the displayed icon.
I'm planning to use this in the window headerbar in a set of buttons
designed to controll testcase filter state.
Signed-off-by: Anna Schumaker <anna@nowheycreamery.com>