* Changed database to introduce Citations
* Introduced new Citation Primary object and CitationBase (equivalent to CitationRef) child object
* Implemented CitationTreeModel and CitationListModel
* Implemented CitationTreeView and CitationListView for new citation views in the navigator
* Implemented EditCitation which is used both for the citation views in the navigator and for the citations of an object
* Implemented the CitationEmbedList to display the citations of an object
* Modified the bottom bar gramplets to support citations
* Implemented the citation selector.
* Modified Media object to include references to Citations
* Initial work on deleting citations
svn: r17960
bsddbtxn.py -- Wrapper for BSDDB DBTxn to support context manager protocol
txn.py -- GrampsDbTxn class to manage atomic transactions
write.py -- GrampsDbWrite class for read/write databases
read.py -- GrampsDbRead class for read-only databases
undoredo.py -- GrampsDbUndo class to manage the undo database
upgrade.py -- Helper module for upgrading a database at open time
svn: r13078