Define functional Flatpak App manifest

This manifest allows for the Gearsystem application to start and execute
ROMs without hanging or being unable to render windows.

The `--share=ipc`, `--socket=x11` and `--device=dri` arguments along
with the GLEW and GLU shared modules are necessary for the GUI to appear
whereas the `--socket=pulseaudio` and SDL2 shared module are necessary
for audio to be processed without hitting an indefinite lock.

Filesystem access is still in development, meaning that the
`--filesystem=home` or some equivalent command is required to find ROMs
to load into Gearsystem.

.gitmodules

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+[submodule "platforms/flatpak/shared-modules"]
+	path = platforms/flatpak/shared-modules
+	url = https://github.com/flathub/shared-modules.git

platforms/flatpak/.gitignore

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+#Ignore any files created during the build of the Flatpak application
+.flatpak-builder/

platforms/flatpak/io.github.drhelius.Gearsystem.yml

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+id: io.github.drhelius.Gearsystem
+runtime: org.freedesktop.Platform
+runtime-version: '23.08'
+sdk: org.freedesktop.Sdk
+command: gearsystem
+finish-args:
+  - --share=ipc
+  - --socket=x11
+  - --device=dri
+  - --socket=pulseaudio
+modules:
+  - shared-modules/glu/glu-9.json
+  - shared-modules/glew/glew.json
+  - shared-modules/SDL2/SDL2-no-libdecor.json
+  - name: gearsystem
+    buildsystem: simple
+    build-commands:
+      - make
+      - make install prefix="$FLATPAK_DEST"
+    subdir: platforms/linux
+    sources:
+      # A reference to a specific commit is used whilst this is in development.
+      # Ideally this will use either a release archive or a specific tag/commit
+      # pair.
+      # There is also an option to use a type of "dir" and a path of "../.." to
+      # allow for builds from the current commit. This may be useful for
+      # "master" or "nightly" builds, but this may require further exploration.
+      - type: git
+        url: https://github.com/drhelius/Gearsystem.git
+        commit: 0fda5894b310110532a5594b815c20cf9a6cb1ed