Fix #35461: Marker gets initialized to arbitrary position

Use center of currently visible frame part instead of
center of the whole frame for position of marker which
is adding from toolbox.

Used separate operator for this to keep operators more
atomic and not confuse with lots of conflicting properties.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56959 954f8c5b-7b00-dc11-b283-0030488c597c