Fix regression in text selection

While 'button' should officially be none on mouse move events, the
aura and windows events set 'button' on mouse move, and selection code
appears to depend on it.

Change-Id: I49f84e6f9178c3b2cb0f2c2c8a7b1d30141d0b4e
Reviewed-by: Michael Brüning <michael.bruning@qt.io>