I have implemented 2 features requested in Ticket #841.

    The user can now specify an arbitrary number of file-link columns in te "Entry table columns" preferences tab, to be added to the main table. The user can any file types specified in the list of External File Types. The extra columns work exactly like the original file-link column.

    For each file-link column whenever there are multiple files, the letter "m" is displayed at the bottom right corner of the icon.

 Committer: noravanq

 On branch master
 Changes to be committed:
   (use "git reset HEAD <file>..." to unstage)

	modified:   src/main/java/net/sf/jabref/GUIGlobals.java
	modified:   src/main/java/net/sf/jabref/JabRefPreferences.java
	modified:   src/main/java/net/sf/jabref/TableColumnsTab.java
	modified:   src/main/java/net/sf/jabref/gui/MainTableFormat.java
	modified:   src/main/java/net/sf/jabref/gui/MainTableSelectionListener.java
	modified:   src/main/java/net/sf/jabref/gui/PreventDraggingJTableHeader.java