FileChooser can go to the background

[LinusDietz]

Latest JabRef, OSX

Steps to reproduce:

1. In the General tab of the entry editor: Click on the + to add a file.
2. Select/Alt-Tab any other programs
3. Switch back to JabRef.

Now it might be (sometimes) that the File Chooser window is behind the Main JabRef Window.

Fix: Ensure that this FileChooser is ALWAYS in the foreground.

[Siedlerchr]

This is not always the filechooser, I experienced this with the "Backup databse found dialog, too", that starts with. But If I remember correctly, the filechooser should already be a modal window. I think that's a thing not really from our side?