Make it easier to describe sequences

[jonrkarr]

• Insert a button “select residue” below the box where you enter the sequence. This button would open a window with clickable names/images of (modified) residues.

• Insert a button "create residue" which opens a ChemAxon Marvin interface where one can draw a residue, select bond/displaced atoms and specify other attributes. (Although, I guess this point is probably too tricky to implement and comes with licensing issues.)