Add basic label component

Labels are meant to represent the text labels added to diagrams like helix
labels, organism names and other text annotations. This is a catch all
mechanism for such labeling.
Remove unneeded config option
Add outline of methods to draw labels
Rename labels to helixes

We are going to be labeling helixes not just generic text. This adds some extra
assumptions, such as we always have a list of nt ids associated with the label.
I can add a generic label component later if needed.
Updating data

We are now going to use some data that has helix annotations. This involves
changing the nucleotide positions as well as adding the helix location data.
Load helix data

We load without the plugin for now. Later we will load with it.
Basic attempt at adding helix labels

In the case of airport diagram we simply put the labels where we are asked to.
We can't correctly compute a location. In the case of a circular diagram we
place the labels at the first base in the helix. This is limited and not the
best thing to do long term. However, we are starting with it to have something
working.