Outliner: Move mode toggling to left column

Add a column of icons in the left gutter of the outliner for controlling
the interaction modes of objects. When an object is in a mode other than
object mode, the mode icon will draw to the left of that object. Any
other objects that are valid to be added or swapped into the mode are
drawn with a dot to the left of the object.

Clicking the dot to the left of an object will swap that object with the
current active object in the interaction mode. For edit and pose modes,
ctrl clicking the dot will add that object to the current mode.

Clicking the mode icon next to the active object removes it and all
other objects from the current mode.

The behavior is nearly identical to the previous edit/pose mode toggling
by selecting the mesh and armature datablocks, with additional support
for all interaction modes.

Currently two undo steps are pushed to prevent an assert.

Part of T77408

Manifest Task: https://developer.blender.org/T68498

Differential Revision: https://developer.blender.org/D8641

source/blender/editors/include/UI_interface_icons.h

@@ -106,6 +106,7 @@ struct PreviewImage *UI_icon_to_preview(int icon_id);
 int UI_rnaptr_icon_get(struct bContext *C, struct PointerRNA *ptr, int rnaicon, const bool big);
 int UI_idcode_icon_get(const int idcode);
 int UI_library_icon_get(const struct ID *id);
+int UI_mode_icon_get(const int mode);
 
 #ifdef __cplusplus
 }