add metainfo and .desktop file

[gmankab]

This Pull Request related to #2810

It changes the following:

• add metainfo file
• add .desktop file

these files are required to publish app on flathub

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[gmankab]

@cruessler
@extrawurst

sorry for pinging, but can you please review this?

[cruessler]

@extrawurst I can have a look if this is something we want to get merged.

@gmankab I saw that the linked issue contains a few existing projects for reference. Would you be able to provide a link to a spec as well?

[gmankab]

manifests:

• https://github.com/flathub/com.helix_editor.Helix
• https://github.com/flathub/io.neovim.nvim
• https://github.com/flathub/org.vim.Vim

manifest that i wrote for gitui:

• https://github.com/gmanka-flatpaks/io.github.gitui_org.gitui

metainfo:

• https://github.com/helix-editor/helix/blob/master/contrib/Helix.appdata.xml
• https://github.com/neovim/neovim/blob/master/runtime/nvim.appdata.xml
• https://github.com/flathub/org.vim.Vim/blob/master/org.vim.Vim.appdata.xml

[gmankab]

@cruessler
@extrawurst

sorry for pinging again

any news?