Add libudev support

This adds optional libudev build system integration and the helper class
"UdevContext" for for initialisation and (thread safe) access to the
udev context.
Add libinput support

Add support for Wayland's libinput as an alternative to the existing
linux input code.
Merge branch 'libinput' of git://git.xcsoar.org/xcsoar/felix/xcsoar