Inconsistent casing in MDM's menuitems

[aminomancer]

Hi, this is a minor nitpick but worth fixing. Firefox uses title case for all menus/menuitems, and sentence case for toolbar buttons. But Simple Translate makes some menuitems that have sentence case labels like Open in sidebar or Open in new window so there's a mild sense of jank when using the context menu. They should be changed to title case (e.g., Open in Sidebar and Open in New Window) to be consistent with Firefox's conventions. (see Firefox's menuitem Open in New Window for example)

Thanks 🙏