BPM indicator

[devmount]

Is your feature request related to a problem? Please describe.
When starting to play a song, it's sometimes hard to get the correct speed right away.

Describe the solution you'd like

• Songs should hold an original bpm value
• The performer should be able to save a custom bpm value per song
• There should be a little blinking light indicating the speed
  • in setlist presentation mode
  • in song fullscreen mode
  • next to the number input when editing the song

Additional context
Dependend on #201. Reported by @EinEnton.