Add flatpak manifest

[qnixsynapse]

Describe your changes

Add flatpak manifest for building flatpaks on Linux

Refer: #698

TODOs:

• Create flatpak manifests
• Add circleci tests
• submit to flathub

Testers and reviews welcome! You will need org.kde.Sdk 6.5 and org.freedesktop.Sdk.Extension.node14 for it to build. Please do install it before running flatpak-builder.

[cosmic-snow]

I've tested this PR on native Linux (v2.4.19 / Linux Mint 21.2) and it has been working fine for me.

Things tested so far:

• building, local (i.e. user) installation
• changing the model directory to new one, non-standard (Flatpak has its own default dir and I wanted to try a different one)
• downloading a model in said directory (Orca Mini Small)
• had to restart after download, but then the model loaded fine
• GPU and CPU ran fine

I think having to restart sometimes after a model download is not uniquely a Flatpak problem, I've seen it mentioned by others in the past, too.

[apage43]

If you have flatpak-builder older than 1.2.3 (most recent on ubuntu 22.04 is 1.2.2) you'll hit the bug fixed here flatpak/flatpak-builder#497 - there's a workaround in the linked issue (but it bypasses a git vulnerability mitigation)

[apage43]

works here too

[apage43]

model support is a bit weird right now - latest versions do not support models that were requiring the old llama.cpp forks in order to not break support for old files (gptj, mpt), and only llama is currently supported with Vulkan - we're currently working on updating our llama.cpp to a post-GGUF version before re-adding models so that we can unify all the file handling logic and ideally match file formats with any other ggml implementations of those models (that have also updated to GGUF)

sideloading was always tricky for non-llama ggml models as there's not a single "ggml format" for many model types that were implemented outside llama.cpp as different implementations wound up making different decisions

[qnixsynapse]

I copied @Tim453 's appdata and made some changes.

Feel free to change it according to your liking.
Also we will need some up to date screenshots.

[ngtanthanh]

.