fix for issues #5 and #12

New cells without cell group didn't appear in the app because no refresh was done when going back. The first patch is a quick fix that simply assigns uncategorized cells to "My Worksheets" which already exists, so they are shown when touching "My Worksheets" a second time (ignore this). The second patch forces a refresh when going back to the cell list. The third patch forces a refresh of the main CellGroupFragment so that any new group added with new cells appears in the list.

As patches 2 and 3 fix issues #5 and #12, there is no longer need for patch 1.