Add a method to change the annotation element group with a single keystroke or click

[manthey]

Currently, to change an annotation element's group, you need to use the context menu which requires at least two clicks; for speed, we could bind a hotkey which would change the element under the cursor to the currently selected group.

[manthey]

It was recommended to add an annotation mode called "Set Group" which, when active, would set the current annotation element under the mouse cursor to the current group.