Only enable the filepath icon if there are any filepaths to show

See trac: https://trac.openmicroscopy.org.uk/ome/ticket/12298

To test:
- Select an omero4 image (e. g. user-3, Project-user-3/archivedv/...CFPNEAT01_R3D.dv.log): "Show file path" icon should be disabled 
- Select an omero5 image (e. g. user-3, Project-user-3/test/Beta Catenin.lif [Image000]): "Show file path" icon should be enabled, and clicking on it should open the "Show file path" menu; clicking on that should open the dialog showing the file path.