qt: fix contrib to avoid race conditions and make it work on Linux and Macosx reliably

The Qt packages were patching each other at contrib time, which made them failing if one of them was being rebuilt. At the same time, it was adding a lot of complexity in the contrib rules.

Instead, let them build and install the plugins normally, but force qmake to provide a pkgconfig file for each of them, and fix/link those plugins from configure.ac.

To achieve this in configure.ac, provide a test that check for QT_STATIC to enable the addition of static plugins.

In addition, fix vlc-qt-check to integrate the plugins when building statically.

Finally, fix some issues when building the Qt contribs for Linux, and add support for the XCB-related plugins and Wayland plugins. No specific choice is made on the available compositor plugins for Qt at the moment, I'm still not sure if we need all of them or just choose one among them.

I still have some cleanup to finish in the commits, some checks on MacOS to remove some code in contrib/ if possible, maybe some improvement in the way we add the plugins (maybe we should always use LIBS_qt and CPPFLAGS_qt to simplify both binary targets) before the merge request is ready for merge.