demux_playlist: add mov RTSPtext tag parser

The quicktime html scripting guide suggests to wrap urls not
necesarly associated with quicktime in a .mov file.
(so that when <embed>ing videos quicktime would be forced.)
These mov files may contain several "Text Hacks".
One of these is RTSPtext.

The suggested/allowed format (as regex) is like:
  RTSPtext[ \r]RTSP://url
See also p.51 of:
https://developer.apple.com/library/mac/documentation/QuickTime/Conceptual/QTScripting_HTML/QTScripting_HTML.pdf
In reality there are also files like (e.g. zdfmediathek.de):
RTSPtext\nrtsp://url\n\n

Lets handle these files as a playlist with one element.
gl_common: signal to GL backend whether we are probing

This is supposed to reduce the amount of useless error messages shown
during initialization of vo_opengl. If multiple backends are compiled,
usually only one of them will work. For example, on Linux both X and
Wayland backends can be compiled, but usually either Wayland or X is
running. Then, if Wayland is not running, but X is, trying to initialize
the Wayland backend should not spam the terminal with error messages.

Code using this should probably raise the log level for error messages
from MSGL_ERR to MSGL_V. Only error messages that are typically printed
in these situations should be changed. For example, if Wayland is
available, but some other part of initialization fails, the error
messages should probably be visible.
x11_common: probing

Not too sure about this. It sucks a bit that I had to add vo.probing.
wayland/common: probing
gl_common: add wayland backend before x11 backend
x11/common: print message according to probing status
wayland/common: print message according to probing status
Removed duplicating message-call in x11/common and wayland/common
wayland/common: use MP_MSG rather than mp_msg