[FEATURE] [Package] Developer for Flatpak

An update on the Flatpak build. I'm working on it. However, it's also possible that there won't be any support for Flatpak. The reason for this is that I have much more to do than spend all my time dealing with Flatpak builds, dependencies, etc. It's simply better to build Snaps here because:

• The way Snaps are built is simpler and more accessible.
• Package dependencies are easier to obtain through stage packages (e.g., wget/curl/xz-utils, which I have to build manually in Flatpak).
• Problems with tools that come from different runtimes (e.g., Heimdall needs QT5 from the freedesktop runtime and Fastboot Assistant needs the Gnome runtime).
• Missing permissions. I would rather spend my time developing the ATL UIBase, the Fastboot Assistant code, or other projects than spending hours creating solutions for dependency problems.

Someone else is welcome to take over creating the Flatpak, administering the Flatpak, etc. My attempts can be found under the commits (not tested):

• 33f7115

• bfedb65

Informations:

• for program version: v.0.9.x or later
• for OS: Flatpak
• section: package