Tweak flowbox selection behaviour

Tweak flowbox selection behaviour

Make flowbox selection more orthodox. Control and Shift now
modify the selection behaviour pretty much in the same way
they do in a tree view, and clicking without modifiers will
clear the selection.

When dealing with touch devices, we treat modifier-less events
as modifying the selection.

This commit also adds a few other selection necessities, such
as catching Escape key presses to cancel rubberband selection,
and handling grab notify.