[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 applicable name.

How it looks now with only removable devices shown:

And with all devices shown:

cc @teams/vdg