Add a way to sort and filter by battery status

[helje5]

Not sure it provides any actual extra value, might just complicate the (intentionally simple) UI unnecessarily.

IIRC the list is currently sorted by low-bat (i.e. low-bat devices should always appear at the top), then by type & name.

But we'll see, a sort-popover button is probably easy to do.

[helje5]

More feature requests:

• sort by name, room, battery status, function

For room/function we'd probably need to group the devices.