Adding some small features

[nchenche]

• add clear BILN button
• add undo / historic functionality
• add monomer symbol info on monomer item sequence hovering
• allow multiple 3d structure representation (e.g. stick + cartoon)
• add structural restraints (SS constraints, 3D template...)
• add protonation state handling for smiles (and pdb?)
• add function to mutate/replace monomer item (analogous or not, ...)
• add function do delete a sequence