Manipulator: experimental lamp positioning tool

- New manipulator tracks lamps to position under cursor.
- Works with multiple lamps, keeping relative offsets.
- Holding Ctrl moves the lamp.
- Access via manipulator or Shift-T.

Code could be improved, but like to get feedback from users.

source/blender/editors/transform/transform_ops.c

@@ -1050,6 +1050,8 @@ void transform_keymap_for_space(wmKeyConfig *keyconf, wmKeyMap *keymap, int spac
 			kmi = WM_keymap_add_item(keymap, "WM_OT_context_menu_enum", TABKEY, KM_PRESS, KM_SHIFT | KM_CTRL, 0);
 			RNA_string_set(kmi->ptr, "data_path", "tool_settings.snap_element");
 
+			/* Will fall-through to texture-space transform. */
+			kmi = WM_keymap_add_item(keymap, "OBJECT_OT_transform_axis_target", TKEY, KM_PRESS, KM_SHIFT, 0);
 
 			kmi = WM_keymap_add_item(keymap, OP_TRANSLATION, TKEY, KM_PRESS, KM_SHIFT, 0);
 			RNA_boolean_set(kmi->ptr, "texture_space", true);