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.

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 PTP camera.

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