Add KDE Matrix icon + replace template room png with svg

[Steinhagen]

The KDE Matrix room icon was added as a layer to the room template. The colors used were selected to be unique and as close to the KDE project as possible.

This commit also replaces the default Matrix room globe png with it's correct svg image taken from their SDK.

[Lassulus]

the planet still looks kinda weird, but it's surely a huge improvement :)

[SigmaSquadron]

This PR was very messy and edited a bunch of things it really shouldn't have. A normal-looking matrix icon PR would be #136 or #137, which just adds the new layers and doesn't re-format the entire file. Both of these now have merge conflicts because of the new formatting.

A good way to prevent Inkscape from doing this is to use the Nixpkgs revision that builds the icons to run Inkscape, and then manually patch out the few formatting changes that the old version of Inkscape makes.

Well, I guess the silver lining is that we'll have smaller diffs when the file is edited with a modern Inkscape going forward.

[Steinhagen]

I wasn't aware that we should've used the 'matrix/pkgs.nix' Inkscape version. Will the revisions in that file be updated to reflect the version used for our svg? Also, is there any documentation on how to patch the formatting changes? If there isn't, I think it would be wise to have this documented somewhere.

[SigmaSquadron]

Yeah we probably should have a README for matrix icons. Samuel used to manage this repo, but since he left Nixpkgs, the knowledge of how to properly add new icons has gone with him. The old inkscape thing is pretty much a hack.