Fixes #24: Extract Molecule feature added to ThumbFish context-menu for multimol files such as SDF, RDF, CML and SMI. Molecules from the files can be extracted in their own contained format such as MOL for SDF or to other formats such as CDXML, EMF, PNG etc.

This is a first set of changes and more changes/fixes will follow for this feature.
Bug fixes and re-factoring for Extract Molecules command.
Changes to run the UI commands on a separate thread to avoid any weird behaviors due to holding the context-menu execution.