flatpak: Add flatpak manifest for gnome-latex

Adds a flatpak manifest for gnome-latex, which seems to function properly. This uses the new org.freedesktop.Sdk.Extension.texlive extension in flathub to solve the issue of how to handle including texlive.

Unfortunately, gnome-latex does not compile against the latest version of vala, which is used in the SDK, so I had to bundle vala with the app. This also required a patch to stop building valadoc, and thus remove a dependency on graphviz.

As the extension used is for the SDK, rather than the platform, this app has to use org.gnome.Sdk as the runtime, otherwise the extension is not loaded. Additionally $PATH is modified specifically for x86_64-linux, however as the texlive extension is only compiled for x86_64 this app wouldn't be able to run using this extension on other architectures anyway.

Also moves the icon name to org.gnome.gnome-latex in order for flatpak to pick it up properly.

Closes #39