[applets/devicenotifier] Move all settings into main UI itself and rename

Right now the Device Notifier applet has two settings: what kind of
devices to show, and whether or not to open on new device attachment.

My sense is that nobody knows that these options exist because they're
hidden away in a config window that you can only access by
right-clicking on the item in the system tray.

Furthermore, the setting for what kind of devices to show is more useful
than we give it credit for; when set to "All devices", the applet functions
as an always-visible way to access all of the machine's disks. This is
quite handy.

Accordingly, this commit removes the config window and moves all of the
settings into the main UI of the applet using PlasmoidHeaders--one
header and one footer, exactly like the plasma-pa applet.

With this change, the "Show all devices" feature becomes discoverable:
when you plug in a flash drive or whatever, the applet will pop open,
showing you the combobox that chooses what it will display. In this moment,
you can notice it, play with it, and configure it to show all devices if
you'd like.

Accordingly again, the name of the applet is renamed to "Disks &
Devices" since "Device Notifier" is not an accurate name. It was only
ever accurate when using the applet's default settings, and now that
non-default settings are made more discoverable, it feels appropriate
to give it a more generally appropriate name.