closes #11

Nix: ignore earlier build results

Allows `nix-build` for basic building and `nix-env -f . -i` for installing.

lmdbxx requires support for thread_local which is not available on that clang version.

- Adds detection for duplicate events

closes #46

Basically reverts the last font related commits since pointSize isn't
as reliable as pixelSize.

Also some layout values (margins, spacings) have been moved out to Config.h.

- Add icon set on mac for future packaging

* Validate both inferred and explicitly entered server addresses by attempting to call the /versions endpoint
* If the domain from the mxid fails validation, try prefixing it with 'matrix'
* Only show server address field if address validation ultimately fails

- Fixes small font issue on MacOs