issue #7: Do not call "close projects" within EDT

use SwingWorker instead
update version to 1.3.1 and update merged-in bugfix
issue #6: Remove "Close all/other projects" from main menu because of NB8

* Remove from File menu -> Removed (still available via context menu of project node)
* But keep them in project's context menu, simply delegate to native action -> NO, code is kept, because this plugin will be available for 7.x and I do not want to create a separate plugin for 8.x
update doc with merged-in bugfix
updated screenshot
updated description
provide package 1.3.1