Added an option to reset file selection on a mouse click without modifiers

The option "A plain mouse button resets selection" reset selection
on a mouse click without modifiers (i.e. Shift or Ctrl).
Selection with Shift/Ctrl for the mouse button have to be enabled
for this setting to work.

ADDED: Option to reset file selection on a mouse click without modifiers
GUI: Added Konfigurator option for resetting file selection

Discussion: !25