Importdialog closeable

[dominikl]

Prevent to close the ImportDialog tab. Fixes #181 .

Test:
Right click on the "Select data to import" tab. Without the fix the "Close, Close All, etc." dialog will show up. If you close the tab via this dialog, close the importer dialog and open it again, it'll crash with the IllegalComponentStateException. With the fix you shouldn't have the possibility to close this tab.

[jburel]

• The right-click menu is no longer available for the Select data to import tab
• If I import an image, a new tab is added: The menu is then available as expected.
• If you select Close Others, the Select data to import tab is removed and cannot be brought back
• If you select Close all, the Select data to import tab is removed leading to an exception when re-opening the importer.
• the Select data to import should never be deleted. The implementation of the close method also needs to be reviewed.

[jburel]

• If you select Close Others, the Select data to import tab is not removed
• If you select Close all, the Select data to import tab is not removed but the window does not have the focus. This will be confusing for the user. The user needs to click on the tab to bring the UI back

[jburel]

The problem reported above #195 (comment) is now fixed.
Merging