Load the Avogadro extensions (and file formats)

This extends the plugin loading to load and register extensions and
their file formats. Also removed some dead code that was no longer used.

Change-Id: I4116fb6b4a5065733c679975b44e215cd7269164
Stop generating SMILES, migrate to molecule refs

Stop generating SMILES for the detail dialog, this was the only thing
being generated here and it doesn't help when displaying records on the
web. Migrated the export handler over to using MoleculeRef and the
chemical JSON exporter, in line with what the open in editor handler was
doing.

Change-Id: I1b7893c19453b21da30309e01f1bc80d9723f593
Moved ChemKit use from database to integration class

They are utility functions that make use of chemkit, but MongoChem
should be more neutral on what specific chemical library is used to
generate data for insertion into the database.

Change-Id: I2a7ebd4e8fe3145a98590b211f264457c1de54bb
Moved similarity search code from MainWindow to ChemKit

The ChemKit function simply takes a MoleculeRef for the search molecule,
refs for the set to be searched and a count for the maximum to return.
This moves all chemkit specific code from MainWindow into the ChemKit
integration class.

Change-Id: I056db25d9874a164196d9b4262c958a78be36328
Added ChemKit::heavyAtomCount for indentifiers

Use InChI or SMILES and get a heavy atom count.

Change-Id: Ie12bd3fba76f24acf048dd441317a5ad6aede2ff
Minor code style fixes

Change-Id: I1df795be29c7c6c5bef5671d21a34ff406b35f02
Added back similarity searching on identifier

This is needed for the RPC API.

Change-Id: I70b9100a64f9e08d3fff629a2148a1e3c979c867