load and display a motif list

Mussagl can now load a motif list from a text file in the form
AAGGGCCTT 1.0 0.5 0.0
(The trailing numbers are the red, green, blue channels)
and motifs are properly displayed on the sequence tracks.

I did this by attaching an AnnotationColors color mapper to the mussa
analysis and passing it to the glsequence class for rendering.