Bugfix for issue #46 Deleting folder with Interface files nested underneath, and then saving.

If a tab was open for a certain file and that file was deleted by deleting a parent folder the tab would be kept open. Saving now caused the crash since there was no valid currentDocument any more. Switching tabs causes the currentDocument to be updated and deleting a tab will select another (valid) tab.
Solution: Close the tab as soon as a file gets deleted by deleted parents.