Flatpak Qt/KDE runtime

[Aleix Pol <aleixpol@xxxxxxx>]

Hi,
Recently flathub started building our runtime into their systems. I
recommend using the runtimes there. I updated the wiki to reflect the
change:
https://community.kde.org/Guidelines_and_HOWTOs/Flatpak

You will possibly have to remove and add again the runtime, but other
than that everything should work as usual.

Aleix