Add the ability to switch to a ruler view of the sequence

[cjmyers]

In other part viewers, like on the iGEM website, there is a tab that allows you to switch between part and sequence ruler views. See example here:

http://parts.igem.org/Part:BBa_F2620:Design

Would be good to have the functionality in visBol. It does need to be part of visBOL as this is the only way to allow one to see clearly overlapping features. It is also the conventional way these are viewed in other software.