As far as possible, use per-display debug flags.
This will minimize the debug spew that we get from
the inspector if it is running on a separate display.
We now have a GdkX11Display::xevent signal that gets emitted for every
XEvent and allows you to interrupt processing via TRUE/FALSE return
values.
These return values to correspond to GDK_FILTER_REMOVE and
GDK_FILTER_CONTINUE respectively.
The GDK_FILTER_TRANSLATE case from gdk_window_add_filter() is now meant
to be handled via gdk_display_put_event().
This is in preparation for DND.
It moves a lot of code from gdkclipboard-x11.c to
gdkselectionoutputstream-x11.c to untangle it from GdkX11Clipboard
usage.
This object tracks the SelectionNotifyEvent that has to be sent in
response to a SelectionRequest.
Currently it just looks like code reshuffling, but it's a prerequisite
for handling MULTIPLE, which requires to only send the notify after
every stream has writtten at least once.
But anyway, code is cleaner now, so it's a win!
