fist changes for #309

- created a button "Always ignore duplicates" duplicates in the DuplicateLinkDialogPanel
--> if pressed: set a booleanProperty and simply always return the first matching result (if there is one) instead of showing the dialog and canceling or resolving

TODO:
- re-implement cancel action
- test if this works for all invokes in the code
(re)set duplicate ignoring in preferences (#309)
bugfixes + dead code removed #309

and removed a warning that appeared after drag & drop of URLs (e.g. http;://google.com) from JabRef to mind map node