flatpak - allow to add/remove multiple flatpaks at once (#2521) (#2822)

* reviving flatpack PR

* added changelog fragment

* adjusted integration tests per PR

* adjusted examples to use the full name of the module

* Use new local artifacts.

* Re-add StrictVersion import.

* Try to clean up PR.

* ...

* Use original name in installed/not installed list.

* More fixes.

* Work around flatpak bug.

* Fix bug I introduced.

Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 3997d5fcc8)

Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>

tests/integration/targets/setup_flatpak_remote/create-repo.sh

@@ -18,7 +18,7 @@ flatpak install -y --system flathub org.freedesktop.Platform//1.6 org.freedeskto
 # Add individual flatpaks
 echo $'#!/bin/sh\necho hello world' > hello.sh
 
-for NUM in 1 2; do
+for NUM in 1 2 3; do
     flatpak build-init appdir${NUM} com.dummy.App${NUM} org.freedesktop.Sdk org.freedesktop.Platform 1.6;
     flatpak build appdir${NUM} mkdir /app/bin;
     flatpak build appdir${NUM} install --mode=750 hello.sh /app/bin;