In all Editors, it should be possible to deselect by clicking in an empty area.
So far we support this in:
- [X] 3D viewport
- [ - [X] UV/Image Editor
- [X] Outliner
- [X] Sequencer
- [X] Node Editor
- [X] Clip Editor
- [ ] Dopesheet
- [ ] Graph Editor
- [ ] NLA