window: ignore skip-taskbar hint on parentless dialogs

Dialogs that don't have a parent should not be skip-taskbar,
otherwise they get lost and there is no way to recover them
(because they're not autoraised when activating the parent),
but toolkits and applications set the hint anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=673399
background: don't save pixbuf in user data

https://bugzilla.gnome.org/show_bug.cgi?id=706777
idle-monitor: Fix a warning when a callback removes the user active watch

The user active watch is a one-fire watch, but it is valid in the API
for the callback to explicitly remove the watch itself. In that case,
the watch will be invalid after the user removes it, and the memory
potentially freed. So make sure to not dereference the watch after
the callback is called.

https://bugzilla.gnome.org/show_bug.cgi?id=706825