Port most of the player to GTK4. Some things are still broken or disabled due to GTK change, but will be gradually fixed.
Initial meson build system support. We do not create .desktop file yet, but we need an option to open media files from GUI first anyway.
I know that this should be done using some sort of build system (like meson), but the player is still far from finished and a basic install script should be sufficient for the time being, if anyone wishes to test it.
