T.EEG.PlotSpect initial playing
Sort of proper eeg loading and plotting

splits eeg data into chunks and shows mean power spectra  over all
chunks.  Doesn't do any checks on eeg header before using it and doesn't
display eeg file in file info pane.
restructure & feature  - see details...

restructure - T.modeChangeCallbacks array now used for modules to
register interest in changes of the mode (i.e. drift on/off).
feature - speed hist now renders for drift too...the slivers of each bar
are coloured acording to the time of the pos sample contributing to that
bit of the bar..with the pos samples sorted within the bar.
improve - speed hist painting

now has half-pix like regular plotting
improve - drop zone prettier and returns later
disable - eeg file loading

until we can display eeg file bricks and better spectrum plot