Introduce synchronize merge method

* Create history-based merging that keeps older data in history instead of discarding or deleting it
* Extract merge logic into the Merger class
* Allows special merge behavior
* Improve handling of deletion and changes on groups
* Enable basic change tracking while merging
* Prevent unintended timestamp changes while merging
* Handle differences in timestamp precision
* Introduce comparison operators to allow for more sophisticated comparisons (ignore special properties, ...)
* Introduce Clock class to handle datetime across the app

Merge Strategies:
* Default (use inherited/fallback method)
* Duplicate (duplicate conflicting nodes, apply all deletions)
* KeepLocal (use local values, but apply all deletions)
* KeepRemote (use remote values, but apply all deletions)
* KeepNewer (merge history only)
* Synchronize (merge history, newest value stays on top, apply all deletions)

src/cli/Merge.cpp

@@ -23,6 +23,7 @@
 #include <QTextStream>
 
 #include "core/Database.h"
+#include "core/Merger.h"
 
 Merge::Merge()
 {
@@ -82,7 +83,8 @@ int Merge::execute(const QStringList& arguments)
         return EXIT_FAILURE;
     }
 
-    db1->merge(db2);
+    Merger merger(db2, db1);
+    merger.merge();
 
     QString errorMessage = db1->saveToFile(args.at(0));
     if (!errorMessage.isEmpty()) {