Make 'Input path' text fields easier to complete for novices

The 'specify path' text fields in the preferences for Chimera, Hmmer, etc look like regular text input boxes, and for novices it is not obvious that one may also double click them to open a file chooser.

* it needs to be more obvious that double clicking will open a chooser
* even more convenient would be if it were also possible to drag and drop executables/directories