Add bluetooth on/off toggle switch

[pappasadrian]

Right now, if bluetooth is off, the "Turn on" menu item needs to be pressed, which closes the menu entirely, resulting in more clicks to reopen the menu and choose the device to connect to. It would be better if the bluetooth can be toggled on and off directly from the menu as well, for quicker toggling.

[smcv]

The Turn On menu item is actually provided by upstream GNOME Shell, rather than by this extension.

It's no longer present with GNOME 43's Quick Settings menu (there are now separate click targets for toggling Bluetooth or opening a menu of options provided by this extension), so any workaround in this extension would need to be limited to GNOME 42 and older.