Add Quadrix

Note: This is mainly marked as draft since I hope that some of the issues described in the notice will be fixed in near future as the author seems quite active. Upstream issues 1 (fixed!),2. Submitted it here already for proofreading :)

Quadrix is a cross-platform ReactXP-based Matrix Client, currently only available on Flathub AFAIK. Works well on mobile, there are only some caveats since its Linux version uses Electron which leads to some troubles. It has an interesting feature called Notepads, which is just creating a room with only yourself in it. But they are viewed a little bit different in the client.

Some remarks:

• Most of the fields were just extracted from the metainfo.xml-file.
• description: It was abbreviated from it original form, added a note to explain that.
• translations: Seems to be done through one file in the codebase, just added a link to it. Not ideal, and I am unsure whether it meets the definiton of link to translation project (from the docs).
• notice: I feel it's pretty long and not very well-written - if you have any ideas on how to make it better and more concise, that would be appreciated.
• screenshots: Upstream screenshots unfortunately only represent the desktop experience. Attached are some screenshots done on ArchLinux ARM, using some test users created on my homeserver.