UI: Click modifier icon to set active modifier

This should be a final piece of the changes for the active modifier
interface. Before, it was necessary to click on the blank space of a
modifier panel to set it active (not the header), this commit allows
clicking on the icon also.

The spacing with the spacing with the expand button would ideally
be a bit larger, but the layout system doesn't offer much flexibility
here.

source/blender/modifiers/intern/MOD_ui_common.c

@@ -307,10 +307,16 @@ static void modifier_panel_header(const bContext *C, Panel *panel)
 
   /* Modifier Icon. */
   sub = uiLayoutRow(layout, true);
+  uiLayoutSetEmboss(sub, UI_EMBOSS_NONE);
   if (mti->isDisabled && mti->isDisabled(scene, md, 0)) {
     uiLayoutSetRedAlert(sub, true);
   }
-  uiItemL(sub, "", RNA_struct_ui_icon(ptr->type));
+  uiItemStringO(sub,
+                "",
+                RNA_struct_ui_icon(ptr->type),
+                "OBJECT_OT_modifier_set_active",
+                "modifier",
+                md->name);
 
   row = uiLayoutRow(layout, true);