Selection.

Very interesting solution!!!
Lets evolve it a bit.
Lets use Timing that we already have.
So if tapping time of Contetextual Selection key is less than click event timing, the tool will perform Exclude Selection else Include selection. In both keyses it will be dropped when you release mouse button.
So if you keep key pressed it will Include Selection or if you hit key and release immediately than start to drag mouse it will perform exclude selection till you release mouse button. So if you hit key and release immediately the next action for mouse drag will be exclude selection.
And make pleas the same rule to work with paint selection to have universal rule for everything, lets experiment a bit.

And Double Hit of Contetextual Selection key will clear selection.
Very interesting 

I don't think it's a good idea to go any further. Otherwise things will get too complicated and troublesome.

Good

smardac, you asked for disabling switching to other modes from object mode if no object selected. Now I have changed it back. Because some people go to subobject modes without any object selected just for retopo.

http://www.digitalfossils.com/Download/NVil-Feb-07-16.rar

Aaa, ok.