[notifications] Remove Qt 4 support.

Not needed anymore, and an upcoming change is Qt 5-only.
[notifications] Use Q_GLOBAL_STATIC_WITH_ARGS.

This (unlike QScopedPointer) has proper lifetime-control, i.e. it will destroy
at the right time before libraries are partially destroyed.

This was causing crashes with some uses of the notification library.
Merge pull request #6 from rburchell/master

Fix a crash in some cases with notification library use.