This is simpler given we only have a couple of windows with these looks,
and removes the dual mode of the ToolbarWindow class.
We just draw the title into the window frame and rely on CSS reserving
enough space (exposed as a new -moz-mac-titlebar-height environment
variable).
We remove the toolbox and toolbar appearance values on mac, now that
they do nothing (toolbar did, but it didn't support dark mode and is
effectively unused).
Differential Revision: https://phabricator.services.mozilla.com/D205469
ceb5cfe890