Start work on panning
Merge branch 'master' into mousepan
Save & restore action enabled states around mouse panning.

Also, disable (ie. consume) all keyboard events when either panning or
mouse selecting.
Continue preparations for panning
Panning works.  Miiph.
Change the mouse button for following links to button 3.

This is much more convenient for me because I use a tablet on which
right-clicking can be done with a tap but not middle-clicking.  And it
should be better for users of laptops with only 2 physical buttons.