Add delete tree view items to right click menu

[Matthew Shapiro]

Every time I try to delete an element from the tree view, I right click on it and look through the menu. I am then confused for a second why there's no delete option, and then click off so i can hit the delete key.

It's not obvious the delete key is even usable and there should be a more discoverable way to delete tree view items.

[Victor Chelaru]

Added delete option on right-clicking instances:

Also, if multiple are selected, then the right click menu tells you that:

fix:
2017ad0