Make SiteMenuService more efficient

In its current state, creating or deleting a menu item leads to a reload of the full menu (which can be quite large), while moving or editing items doesn't. This is both inconsistent and inefficient, and should be improved.

Also, the SiteMenuService's API inconsistently requires the client to specify the menu's (UU)ID. For all but the (initial) call to load the menu, this is technically unnecessary. Moving that call from the MenuEditorCtrl's constructor into the HippoIframeCtrl's openMenuEditor method could further simplify the code such that MenuEditorCtrl requires no knowledge of the current menu's ID at all.