xdg,xwayland: raise sub-view correctly relative to other sub-views

xdg,xwayland: raise sub-view correctly relative to other sub-views

When a parent view has multiple sub-views (dialogs) visible, focusing
one sub-view ought to raise it above the others. This doesn't currently
happen -- focusing a sub-view raises the whole group of views together,
but has no effect on the relative stacking order between them.

This seems like a simple oversight in xdg/xwayland_view_move_to_front()
that's pretty easy to fix.

Add FIXMEs to deduplicate this logic in future.

Tested with HomeBank: the Import dialog pops up an additional Open File
dialog, which before this change appears behind the Import dialog (and
clicking on it does not raise it to the front). After this change, the
Open File dialog appears in front as expected.