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/format/KdbxXmlReader.cpp

@@ -17,6 +17,7 @@
 
 #include "KdbxXmlReader.h"
 #include "KeePass2RandomStream.h"
+#include "core/Clock.h"
 #include "core/DatabaseIcons.h"
 #include "core/Endian.h"
 #include "core/Entry.h"
@@ -1032,7 +1033,7 @@ QDateTime KdbxXmlReader::readDateTime()
         return QDateTime(QDate(1, 1, 1), QTime(0, 0, 0, 0), Qt::UTC).addSecs(secs);
     }
 
-    QDateTime dt = QDateTime::fromString(str, Qt::ISODate);
+    QDateTime dt = Clock::parse(str, Qt::ISODate);
     if (dt.isValid()) {
         return dt;
     }
@@ -1041,7 +1042,7 @@ QDateTime KdbxXmlReader::readDateTime()
         raiseError(tr("Invalid date time value"));
     }
 
-    return QDateTime::currentDateTimeUtc();
+    return Clock::currentDateTimeUtc();
 }
 
 QColor KdbxXmlReader::readColor()