Improvement of "Add model resource"

[mrentsch65]

Currently the wizard supports adding picture files with the buttons "Pick vendor logo", "Pick Device Icon", "Pick Device Picture". The usability could be improved by showing the filenames after selection.

A further improvement would be to generalize the functionality by having just a single button "Add model resource" and a subsequent selection of any of the defined submodel types, not only the three picture files. The selected files should then be displayed for overview in a table.