Merged the first bits of Canvas Interaction

[Arjen Hiemstra]

Hi People,

Just to notify everyone, I have merged the first bits of the canvas interaction 
work just in time for the branching of 2.5. This implements the basic actions 
as described in my proposal. They are currently hard coded to the shortcuts 
listed in the proposal, the configuration still needs to be implemented, as 
well as the overlay widgets.

Please test it and let me know what you encounter. Two currently known issues 
are that the rotate and zoom modifiers currently fail when used with middle 
mouse on a tablet and that the pan/rotate/zoom modifiers do not auto-repeat.

Cheers,
Arjen