fixes #270

fixes #87

The color scheme of nheko obeys the default color theme of Qt
(i.e. the system theme). It uses a Qt stylesheet to accomplish this,
which means replacing the color theme with a custom theme would only
be a matter of writing a new style sheet and loading it into the app.

- Fixes high cpu issues caused by the serialization of the whole
  in-memory state.
- Display name changes are now visible in the timeline.

Fixes #95

fixes #14

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.

Minor stylistic refactoring

closes #29

- Add border width on Badge