Flatpak Support

[roland3564]

Is your feature request related to a problem? Please describe.
When I run "install.sh", it says that Krita is not installed, despite it is, I installed it through Flatpak, so I tried to put /var/lib/flatpak/app/org.kde.krita as the directory input, but the program just exits without no errors.

Describe the solution you'd like
I'd like the ability for install.sh to automatically detect the flatpak's directory and doing its need.

Describe alternatives you've considered
I considered trying installing Krita with apt-get, but the program's version is too far behind since the PPA has dropped.

Additional context
I use linux mint 21. the Krita's version is 5.1.1, I have both the stable and nightly version installed.