Added preferences to specify the sort order for saving and exporting entries. Still missing: * Dialog to select sort order * Handling of old sorting preferences ("standard" and "title")
Added new "file sorting" preferences tab. Moved buttons from File Tab to new tab.

Conflicts:
	jabref/src/java/net/sf/jabref/FileTab.java
Added GUI to modify file sorting preferences. Introduced constants for preferences.
Added upgrade of old preferences "saveInStandardOrder", "saveInTitleOrder", "exportInStandardOrder", "exportInTitleOrder" to the new preferences. The old preferences are not removed so that the preferences can be used with an old version of JabRef.
Merge branch 'sorting'