core: GCButtonStyle: minor layout, center button-text
core, DownloadManager: expose status of registering voices

Add getter and signal for the status of registering voices rcc. This
targets for now only voices rccs of the currently active locale.
Generalize if needed.
core: Startup: delay playing welcome voices until voices are available

Without playing the intro music AND activated automatic downloads,
there was a race condition between registering voices rcc and the
playback of welcome.ogg.

We now delay it if voices are not yet available about max. 10 secs.