tabs: rename SelectedTab to SelectedTabContent

This function returns an ui.Drawable. Use a more explicit name. This
prepares for adding a new SelectedTab function which will return
an ui.Tab.

Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Koni Marti <koni.marti@gmail.com>

commands/terminal/close.go

@@ -24,7 +24,7 @@ func (Close) Execute(aerc *widgets.Aerc, args []string) error {
 	if len(args) != 1 {
 		return errors.New("Usage: close")
 	}
-	term, _ := aerc.SelectedTab().(*widgets.Terminal)
+	term, _ := aerc.SelectedTabContent().(*widgets.Terminal)
 	term.Close(nil)
 	return nil
 }