in GUI, revise data import dialog

GUI > Data > +1D:

The file dialog is problematic because there is no correspondence between file extension and data format.

The filters for tabular data are incomplete: besides .csv .dat .tab .txt, we also need .csv.gz etc, and possibly other compressor extensions like zip and bz2. This, however, would make the filter line too long. We rather should use one single filter '*', then either determine the data format from the first line of the file, or launch the csv import dialog.

The csv import dialog should also show the first few lines of the data file.