one thing could for whatever reason not be avoided: Even though the
processors of the caches inside WriteProxy are initialized, they somehow
get "de-initialized" and have to be initialized again...
These are:
* different size of addContact- and SettingsButton
* default icons in light mode for users and groups (even though they are
currently just the version used in dark mode)
* wrong preferred size of unnamed "Login" label in LoginScene
* unopenable LoginScene for some OS (Debian)
* white screen when the current scene is switched
Additionally cleaned up code a bit in MessageControl and
LoginScene(.java)