Close import

Fix 3 issues noticed while testing a PR:

 * Allow the user to close even if he/she has ongoing import. No longer cancel them since we know have notification in the toolbar.
 * Retry All. see http://trac.openmicroscopy.org/ome/ticket/11138
   * Import Mike for example
   * Retry All should be enabled.
   * click on the button
   * Another import should start.
 * Folder and message
  * Select a folder containing a sub-folder containing Mike and Mike.log
  * import the top folder. The Message "File Not valid" should not be displayed