Add Solid backend using libimobiledevice for finding iOS devices

This allows Solid to find them and exposes them as PortableMediaPlayer with
known protocol "afc" similar to how it's done for MTP devices.
Devices get the device name and form factor icon. It also monitors pluggin in
and out devices.

Unfortunately, there's no signal for when the user trusts the device,
so one has to plug it out and back in once acknowledging the prompt.
I don't even get the device name when it's not trusted.

Battery is not provided as UPower already reports this through libimobiledevice.
Also, there's no way for Solid to relate devices from different backends
so iPhones still also show up as camera.

Right now it doesn't do much (apart showing up in solid-hardware5 list
and KInfoCenter's device browser) but is preparing the necessary
infrastructure for improving iOS support throughout KDE.