Fix ValueError with getter returning NoneType

int() and float() do not like having NoneType as an argument.
Massive set of changes to volume, GUI, etc.

GUI: Added playlist, volume, and youtube url add box.

Server: fixed volume bugs concerning mpv complaining a lot when
volume is set when nothing is playing. Also added functionality
for the client to request current song, volume, and other changes
both explicitly and via broadcast.