Edit Mesh: Poly build tool improvements

This commit changes the functionality of the Poly build tool to make it more suitable for retopology tasks:
  - Click and drag from a boundary edge extrudes a new quad
  - Click and drag on vertices tweaks the position
  - Ctrl + click adds geometry. There is a geometry preview in the gizmo. It also can automatically convert triangles to quads.
  - Shift + click deletes mesh elements (faces or vertices)
  - Updated preselection code. Different mesh elements take priority depending on the selected action.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D5573

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -490,12 +490,17 @@ class _defs_edit_mesh:
 
     @ToolDef.from_fn
     def poly_build():
+        def draw_settings(context, layout, tool):
+            props = tool.operator_properties("mesh.polybuild_face_at_cursor_move")
+            props_macro = props.MESH_OT_polybuild_face_at_cursor
+            layout.prop(props_macro, "create_quads")
         return dict(
             idname="builtin.poly_build",
             label="Poly Build",
             icon="ops.mesh.polybuild_hover",
             widget="VIEW3D_GGT_mesh_preselect_elem",
             keymap=(),
+            draw_settings=draw_settings,
         )
 
     @ToolDef.from_fn