player: add `mpv://` protocol

This is useful to run mpv from URL handler. Can be used to start mpv
from you web browser or anything that would trigger url handler for
given protocol. Only safe protocols are allowed.

Current solutions to run mpv in such context are to spawn IPC proxy and
run mpv when requested, but this is in most cases not necessary.

Currently the protocol is added in mpv.desktop. Windows related
registration will follow in future changes.