app, menus: add "Image -> Transform -> Arbitrary Rotation..." action

Add a gimp-rotate-image-arbitrary action, and a corresponding
"Image -> Transform -> Arbitrary Rotation..." menu entry, which
activates the rotate tool in image mode (similarly to the
corresponding action for layers).

(cherry picked from commit 8bff415f59)

menus/image-menu.xml.in

@@ -398,6 +398,7 @@
           <menuitem action="image-rotate-90" />
           <menuitem action="image-rotate-270" />
           <menuitem action="image-rotate-180" />
+          <menuitem action="tools-rotate-image-arbitrary" />
         </placeholder>
         <separator />
       </menu>