On view switch: clear table before doing request for new data

When switching view using the dropdown: When it takes a long time to retrieve the data for the table of the new view it gives the impression that the Ladybug isn't working correctly as you see the data from the old view with the columns of the new view (until the data for the new view is returned).

To reproduce this add the following to the end of the buildMetadataQuery() method in the ProofOfMigrationStorage.java and switch to the Proof of migration view in the GUI:

    try {
        Thread.sleep(10000);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

wearefrank / ladybug-frontend

On view switch: clear table before doing request for new data #257