Merge pull request #114 from diizy/stable-0.4

Graphics update
Integrated new splash screen

Also removed ugly code for displaying translucent splash images.
AutomatableModel: added centerValue property

The optional centerValue property can be used by views to display the
center somehow. It defaults to minimum value.
Knob: draw arc around knob

Initial work on improved knob usability by drawing an arc around knob
indicating current state as well.
InstrumentTrack: set center values for pitch and panning
Knob: draw arc + arc background for all unstyled knobs
EnvelopeAndLfoParameters: added center values for amount models
Knob dots by Tobiasz
Revert knob dots, make knob arc use palette, remove bb-editor image... bye bye TR-808 :(