Bring back the context menu in tree viewer

[maxpatiiuk]

The a11y release removed the context menu due to accessibility concerns and replaced it with buttons in the header.

@tlammer asked to have both buttons and the context menu

[bronwyncombs]

Context menu = right clicking on a node brought up a menu of tree options, e.g., edit, search, add, move, merge, synonymize, etc.

I think this would be nice to bring back especially for split tree view workflows