Set the side of the window buttons in the HeaderBar from the GTK theme

Make two sets of the buttons, one for the start and one for the end of
the HeaderBar, and show one or the other based on the the theme.  Watch
the style context for changes in the theme and update accordingly.  Once
the composer is detached, the default close button is used, so hide both
and stop watching for style updates.  This assumes we never re-attach a
composer.

We also add a bit of space between the separator and the detach button.

https://bugzilla.gnome.org/show_bug.cgi?id=733372