It is possible to cancel a mouse-down without a mouse-up
(for example with an ESC button).
So, cancel an obsolete "MouseSelecting" on mouse-down.

Change-Id: I37d641612cf2cd8099af8f04b12fdb364c4f9e24
Reviewed-on: https://gerrit.libreoffice.org/56857
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>