Patch #35464: Marker placement for motion tracker by clicking on a desired location

Now button in the toolshelf behaves this way:
- User clicks on "Add Marker"
- Then he clicks where the marker should get placed

Patch by Marcos Couto (ocf) with own modifications.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57138 954f8c5b-7b00-dc11-b283-0030488c597c