Blender Git Loki

Add 'EDBM_selectmode_disable_multi' and use in knifeproject

As a followup to rB6f4e595e9ba9 and as per @campbellbarton suggestion,
this adds EDBM_selectmode_disable_multi, similar to
EDBM_mesh_deselect_all_multi & EDBM_mesh_deselect_all, so other similar
uses don't need to be done in a loop.

Also, selected_objects isn't a reliable way to handle this case - since
objects can be in edit-mode & not selected, use
BKE_view_layer_array_from_bases_in_edit_mode_unique_data instead

Differential Revision: https://developer.blender.org/D6317