X11: Don't assume all displays are X11 displays

When iterating over the list of displays gotten from the
display manager, we have to check if what we got is actually
an X11 display.
inspector: Improve tab switching

Switch the object and resource trees away from activate-on-single-click
and add a 'view details' button.
inspector: Improve details buttons

Ensure that the details buttons are only sensitive when
we actually have details to show.