Progress bar and improved speed when associating dropped files with sequences in an alignment

Dropping PDB structures and other files onto an alignment is convenient when working with lots of 3D structures, but Jalview takes some time to import hundreds of structures, and doesn't warn the user about it.

At the very least, the 'AssociatePDBFiles' routine should show a progress bar during import. Ideally the load should happen in parallel (Jmol permitting!).