BUGFIX: Ensure save project task when closing via application close do finished before closing gephi

Improve zip entry handling and null files import.

Fix #1282 Migrate spreadsheet importer to Import API
Fix #1662 Merge CSV and Spreadsheet import

Fix #1699 Add support for excel (xls and xlsx) import
Fix #1461 Spaces in header columns of edge CSV file fails to load
Fix #1661 DNA - import .CSV (from DNA software) in Gephi
Fix #1658 Export data table with large numbers in metrics rows
Fix #532 Suggestions: improvements to spreadsheet import
Fix #1248 Add support for edge kind in spreadsheet import/export
Fix #1431 CSV data missing some information

Sorry for the big merge :)

Remove empty Plugins menu since it's also confusing
Fix other details

* Use diamond inference where possible
* Reorganize imports
* Changed occurrences of String .indexOf() != -1 to .contains()
* Changed occurrences of List .size() == 0 to .isEmpty()

Done with Netbeans inspect and transform feature

#835 Error and file deletion when importing non-gzipped .gexf files (0.8.2-beta, Linux)
#918 Cannot open graph files in /tmp
#757 WTF: Gephi deletes my GML file

Update DesktopImport to the new Graph API. Remove hierarchical graph support and enable crash reports on imports.