LmmsStyle: initial port of scrollbar drawing code from CusisStyle

Colors still need to be made configurable via CSS and tuned for the
default theme.
LmmsStyle: adopted new coding style and renamed class
Merge branch 'stable-0.4' of github.com:LMMS/lmms into stable-0.4
FxMixer, FxMixerView: cache maximum peak until next display update

We lost short peaks because the display update rate usually is much lower
than the number of audio buffers processed per second which lead to lost
peaks. We mitigate this issue by caching the maximum peak value until the
next display update where it is reset.