compiles now, but is not fully tested

instead of determining which component was clicked on, we're now looking fo rthe channel closest to the mouse y.
This allows larger channeloverlap, so there are fewer cases where the data get clipped.

still having an issue where the first sample on the left side of the screen is always drawn as 0, but its not a huge issue for now

20ms work very smoothly on a 1.2GHz core 2 duo laptop, so it's probably a safe value.

LFP traces are now only drawn in the space since the last buffer update, this reduced cpu load.
Reverted back to the linedraw method because speed is not an issue any more with this fix.
Currently not issuing full redraws when a user uses the scrollbar, only for clicks and mouse wheel messages.

used the term 'spread' as in the neuralynx software for now

replaced drawLine method with pixed based plot. Increases speed at the
cost of anti-aliasing.
Also centered channel number display in channel,
and set default channelHeight to 40, so that ~16 channels should fit on
a 1080p screen