keyboard operation of pdb fetcher and pdb structure chooser dialogs

The PDB sequence fetcher dialog supports the standard [tab] keystroke based model to move keyboard input focus between the different elements of the GUI, but the logic does not detect when a search result is highlighted via hitting 'return' or 'space' and so enable the 'OK' button.

Many power users prefer to operate dialogs via the keyboard - this one currently cannot be, and it would be great if it were!