The left handle of the range mover element didn't properly follow
the mouse movement. Also, a lot of code is duplicated between
RangeMover and SelectionRange. This change prepares RangeMover for
being reused in SelectionRange by eliminating external
dependencies.

Task-number: QTCREATORBUG-10762
Change-Id: Ia3b83101263e7af8ed46e1aaf4e92654b0068861
Reviewed-by: Kai Koehne <kai.koehne@digia.com>