Hide tools with missing icons under experimental

This removes from the UI all tools with missing icons and hides them
under a "Tools with missing icons" experimental option.

We agree on not making available by default tools in master without icons.
Having this experimental flag will allow to commit new tools as soon as the
technical design and implementation is finished so development can
continue, without adding broken icons to the UI.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D8831

release/scripts/startup/bl_ui/space_userpref.py

@@ -2164,6 +2164,7 @@ class USERPREF_PT_experimental_new_features(ExperimentalPanel, Panel):
             context, (
                 ({"property": "use_new_particle_system"}, "T73324"),
                 ({"property": "use_sculpt_vertex_colors"}, "T71947"),
+                ({"property": "use_tools_missing_icons"}, "T80331"),
             ),
         )