Implement synchronize merge code added by #1992

[droidmonkey]

The code introduced by #1992 is just backend, but introduces significant use cases. It still requires plumbing to the GUI to resolve the following issues:

• Allow reviewing changes before/after merging #1152 - Allow review of merge changes prior to executing merge
• Right Click -> Merge selected entries #2202 - Right-click -> merge selected entries
• Merging always marks the database as dirty #1010 - Merging always marks database dirty
• Full synchronization on CLI #2184 - Synchronize from CLI

The following are being implemented by #2109:

• Support bidirectional sync with a different database file [$10] #90 - Sync changes from another file (real-time)