[Gtk Config] Port cursor theme sync to KConfigWatcher

Summary:
Port from deprecated dbus signal to KConfigWatcher.

Depends on: D27942

Test Plan:

1. Apply D27942
2. Restart kded5
3. Apply any cursor theme from KCM
4. Check if GTK applications heve the changed cursor

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27943