Supports "dmenu" chooser type, which is called with a dmenu type list
piped to stdin, "simple" type, which recieves nothing on stdin and
default, which tries the hardcoded choosers.
Choosers are required to return the name of the choosen output as given
by the xdg-output protocol.
Thanks to piater for closing overlooked pipes.
Thanks to ericonr for suggestions regarding fork and pipes.
* Initial session support WIP
Remove libdrm dependency
Remove display from context, add dbus properties
Use random names for shm and pw_stream, init the stream only for new cast instances
Separate cast initialized flag from refcount, cleanup names and comments
* Refactor and stability improvements
Properly report xdp screencast implementation version
