test: add iosvlc.m for iOS development

iosvlc.m provides a binary usable as an iOS application, forwarding the
VLC arguments just like VLC on desktop. It allows easier iteration on
vlccore development for iOS, without the need to test in a VLCKit
application like VLC for iOS or new external application.

It is currently designed for usage with dynamic plugins.

To develop with it, you must generate a .ipa archive containing both
the resulting binary as executable, a PkgInfo file, an Info.plist file
describing the package and the libs (libvlc.dylib, libvlccore.dylib, and
every plugin .dylib or additional convenience libraries that are not
linked statically in the Frameworks/ directory. It must then be signed
with a developer certificate allowed by Apple and provisionned with a
mobileprovision file allowing installation on the given device for the
same developer certificate.

Then, tools like libimobiledevice can be used to start the application
with additional arguments or environment variables. They can also be
added in XCode through the "Edit Scheme" menu.

A big part of the iOS-specific code has been originally written by
Marvin Scholz in a more complete libVLC ios sample.

Co-authored-by: Marvin Scholz <epirat07@gmail.com>

Cherry-picked and adapted from commits:
e16f4d9b
0bfe8803
96bee418
904998af
54c392d7