Use sentence case everywhere

We have not been good at consistent casing so far. Now we use 'Sentence case' everywhere. EVERYWHERE.

Also Removing 'Lc' prefix from i18n field names: the 'Lc' stood for lowercase but now that everything
is in 'Sentence case' there's no need for the distinction.

I've got a couple lower case things I've kept: namely, things that show up in parentheses.

pkg/gui/controllers/options_menu_action.go

@@ -37,7 +37,7 @@ func (self *OptionsMenuAction) Call() error {
 	})
 
 	return self.c.Menu(types.CreateMenuOptions{
-		Title:      self.c.Tr.MenuTitle,
+		Title:      self.c.Tr.Keybindings,
 		Items:      menuItems,
 		HideCancel: true,
 	})