Updated ProM import handling

The handling of importing files (be it either through the Import button or by dragging-and-dropping) has been changed.

The import of a file contains two parts; a configuration part and a part that actually does the configured import. The configuration part is required by some files, as some file extension can be imported by multiple import plug-ins. For such files, the user needs to configure the import by selecting the import plug-in to use. For example, there are different plug-ins to import a file with a “xes” extension (an event log).

The first (configuration) part of the plug-in runs in the EDT (Event Dispatch Thread), as the user might be queried about the plug-in to use. The second (configured part) runs in a different thread, to provide the EDT with the opportunity to show progress of the import. The latter relates to an issue mentioned while testing ProM Lite: When importing large log files, no progress  was shown. This was caused because the entire import (both parts) ran in the EDT, which did not offer the EDT to show a progress bar. For this reason, the second part needed to be moved to a different thread.

The change made was to synchronize the second parts in case multiple files are to be imported. This synchronization prevents one import to overtake another. This should fix the problem as mentioned in ticket #3111. As a side effect, one can now get multiple progress bars (one after the other) when importing multiple files.