This PR creates 2 namespaces for VSE code: - `blender::seq` for sequencer core code - `blender::ed::vse` for editor code These names are chosen to not be in conflict with each other. No namespace was used for RNA. Finally, file `BKE_sequencer_offscreen.h` was moved from BKE to sequencer. Pull Request: https://projects.blender.org/blender/blender/pulls/135500