[FEATURE] Allow deselection by clicking with a selection tool

1) Now you can deselect with *any* selection tool:
   rectangular, elliptical, outline.

2) Now you can also deselect with your tablet stylus,
   because there is a small tolerance level for
   considering your new selection as a click. If you made
   a selection that is smaller than 5 (view) pixels, then
   it is considered as a click and the tool deselects
   everything.

3) One can configure the click-tolerance level with a
   config option 'SelectionViewSizeMinimum'

CC:kimageshop@kde.org

# Conflicts:
#	libs/image/kis_algebra_2d.h