src: Add support for right click events

The current implementation treats left mouse button clicks as a
generic "mouse press" modifier, this commit extends the list of
modifiers by adding a "right mouse click" one.

The proper way to implement this would be to ship the coordinates
of mouse key press events in each `Key` object, and pass whichever
button was clicked as a codepoint value (instead of coordinates
currently), but this would require more work.

This commit allows:

* right clicks to set the cursor of the main selection
* control-right clicks to merge all the selections, and then set
  its cursor

Fixes #843

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