Blender Git Loki

3D Cursor: Add option to lock it in place to prevent accidental modification

This option helps users protect themselves from accidentally changing the cursor
location (and not being aware of this until it has already caused problems)
when drawing using Grease Pencil (or with other tools where this is equally likely).
It seems to occur most frequently when using a tablet.

Currently, this only affects the use of the mouse to set the cursor, as this is
where most accidental invocations occur.

(I'm aware that this change may turn out to be quite contentious. Fortunately, it
should be simple to just revert this commit in that case :)