fbd-alert-slider: Small service to track alert sliders

This tracks the alert slider on OnePlus 6/6T devices and adjusts the profile level accordingly emitting a bit of haptic feedback when that happens.

This is heavily based on a script by @isyourbrainFOSS in the Phosh matrix channel and is basically the bits posted there translated to compiled language so we can leverage poll/select and gudev for device selection.

The main differences are

• No need to use polling as we can use a GIoChannel that only triggers when new input is available (thus improving battery life)
• Probe for the device via udev/gudev so we can exit early on systems that don't have a slider (but install the service by default)
• Use important hint on the notification so we can trigger haptic even when in silent mode
• Use a systemd unit for activation (should work on pmOS as AFAIK elogind supports this, otherwise we can add an autostart desktop file)

I've put the whole thing into a separate service as the alert-slider DT bits for the OnePlus 6/6T aren't upstream yet and this way distros have some leeway on whether they want to ship it or not.

@isyourbrainFOSS does that make sense?