Fix for bugs 3741 and 3756. Here's what it does:

Fix for bugs 3741 and 3756. Here's what it does:

      * In the "Available" list, double clicking or hitting Enter adds a
        citation. Hitting Ctrl-Enter adds the citation and closes the
        dialog. Previously, Enter did what Ctrl-Enter now does if there
        was no other citation already selected and double clicking would
        close the dialog after adding the citation.
      * In the "Selected" list, Delete and Backspace delete the
        selection.
        Ctrl-Delete and Ctrl-Backspace clear the whole list.

Thanks to Abdel, Andre, and Lars for comments and help.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18633 a592a061-630c-0410-9148-cb99ea01b6c8