Update ChatViews whenever the stylesheet is reloaded

Update ChatViews whenever the stylesheet is reloaded

UiStyle now emits a signal when loadStyleSheet() (now a slot) is called.
ChatView reacts by invalidating the scene. Note that the ChatLines are not re-layouted,
which means that changes in font sizes and possibly styles might lead to a broken appearance.
This is primarily thought for testing color combinations without restarting the client
all the time.