Use flatpak-spawn to run streamlink when running as a flatpak (#3178)

Co-authored-by: Leon Richardt <leon.richardt@gmail.com>
Co-authored-by: Paweł <zneix@zneix.eu>

src/common/Version.hpp

@@ -29,6 +29,7 @@ public:
     const QString &dateOfBuild() const;
     const QString &fullVersion() const;
     const bool &isSupportedOS() const;
+    bool isFlatpak() const;
 
 private:
     Version();