Apply Modifier: support applying as shape key and keeping the modifier.

This can be useful to save the result of a cloth simulation as a
shape key without destroying the simulation, so it's possible to
e.g. re-run it to get other shapes, or simply use the new shape
key to start the simulation already in a draped state.

It also makes sense to allow applying as shape key even when the
mesh is shared, because the operation itself just adds a shape
key. To support this, split the apply operator into Apply and
Apply As Shapekey so that they can have different poll callbacks.

Differential Revision: https://developer.blender.org/D8173

source/blender/modifiers/intern/MOD_ui_common.c

@@ -231,12 +231,19 @@ static void modifier_ops_extra_draw(bContext *C, uiLayout *layout, void *md_v)
 
   /* Apply as shapekey. */
   if (BKE_modifier_is_same_topology(md) && !BKE_modifier_is_non_geometrical(md)) {
-    uiItemEnumO(layout,
-                "OBJECT_OT_modifier_apply",
-                CTX_IFACE_(BLT_I18NCONTEXT_OPERATOR_DEFAULT, "Apply As Shapekey"),
-                ICON_SHAPEKEY_DATA,
-                "apply_as",
-                MODIFIER_APPLY_SHAPE);
+    uiItemBooleanO(layout,
+                   CTX_IFACE_(BLT_I18NCONTEXT_OPERATOR_DEFAULT, "Apply As Shapekey"),
+                   ICON_SHAPEKEY_DATA,
+                   "OBJECT_OT_modifier_apply_as_shapekey",
+                   "keep_modifier",
+                   false);
+
+    uiItemBooleanO(layout,
+                   CTX_IFACE_(BLT_I18NCONTEXT_OPERATOR_DEFAULT, "Save As Shapekey"),
+                   ICON_SHAPEKEY_DATA,
+                   "OBJECT_OT_modifier_apply_as_shapekey",
+                   "keep_modifier",
+                   true);
   }
 
   /* Duplicate. */