This patch enables displaying the number of entries in each group in the groups tree (as requested in feature request 2868512).
So far, it only works for static groups. It is trickier to implement for the dynamic group types.
Now the patch works for dynamic groups as well.
The implementation may slow down the GUI due to a search trough all database entries for the dynamic group types. Maybe this can be improved by using a worker as seen in GroupSelector.java - anyone, feel free to look into fixing this.
Run 'patch -i patches' in jabref/src/java/net/sf/jabref/groups
Applied in the master branch.