Easymotion-like jump mode

[Omnikar]

Depends on virtual text (#417). I will rebase once that's merged.

I've had to edit the functions for checking whether the primary cursor is in the view to accept any cursor position so they could be used to know when to stop finding jump locations.

To-do:

• Accept a motion to use for jumping (as of now, the w behaviour is used unconditionally).
• Change the color of the file text when in jump mode to make it easier to see the jump location annotations.

Related: #274, #510

[Omnikar]

Replaced by #3791.