New Features: Contextual menus

[tin2tin]

Billrey: "We should also add the contextual menu to the Sequencer too, inside space_sequencer.py. Look at how this is added for the other editors."

https://devtalk.blender.org/t/can-we-in-collaboration-produce-a-working-suggestion-for-improving-the-user-interface-of-the-vse/3264/9