Provide flatpak for QtCreator daily snapshots

Flatpak is a new distribution system for Linux. It depends only on the flatpak application and the Linux kernel. It provides runtimes for X11, Gnome and KDE too, which can be used by every application. Runtime are versioned and can be updated independently to fix security bugs.

Flatpak fixes the binary compatibility problem, the different paths problem but still shares the same binaries(libraries) between different applications. So there will be only one Qt in memory.

It is a package system too, with global and local repositories, even per user, an update mechanism, sharing of binaries and binary deltas for updating.