fixed PDB similar search functionality
will now allow case independent prompts

The thing works now, tried and tested with debugging.

Write "PDB " to activate the extention (uses omnibox)
a dropdown of suggestions pop up with what it can do namely:
"PDB ID 1ASC" will show the structure page of id 1ASC same for any other structure id
"PDB download pdb 1ASC" will directly download the pdb file for the structure
"PDB downlaod cif 1ASC" will directly download the cif file for the structure
"PDB Imatinib binders" will search imatinib binders, the same can be replaced with any other string text