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)

tests/stub/TestClock.cpp

@@ -0,0 +1,86 @@
+/*
+ * Copyright (C) 2018 KeePassXC Team <team@keepassxc.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 or (at your option)
+ * version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "TestClock.h"
+
+TestClock::TestClock(int year, int month, int day, int hour, int min, int second)
+    : Clock()
+    , m_utcCurrent(datetimeUtc(year, month, day, hour, min, second))
+{
+}
+
+TestClock::TestClock(QDateTime utcBase)
+    : Clock()
+    , m_utcCurrent(utcBase)
+{
+}
+
+const QDateTime& TestClock::advanceSecond(int seconds)
+{
+    m_utcCurrent = m_utcCurrent.addSecs(seconds);
+    return m_utcCurrent;
+}
+
+const QDateTime& TestClock::advanceMinute(int minutes)
+{
+    m_utcCurrent = m_utcCurrent.addSecs(minutes * 60);
+    return m_utcCurrent;
+}
+
+const QDateTime& TestClock::advanceHour(int hours)
+{
+    m_utcCurrent = m_utcCurrent.addSecs(hours * 60 * 60);
+    return m_utcCurrent;
+}
+
+const QDateTime& TestClock::advanceDay(int days)
+{
+    m_utcCurrent = m_utcCurrent.addDays(days);
+    return m_utcCurrent;
+}
+
+const QDateTime& TestClock::advanceMonth(int months)
+{
+    m_utcCurrent = m_utcCurrent.addMonths(months);
+    return m_utcCurrent;
+}
+
+const QDateTime& TestClock::advanceYear(int years)
+{
+    m_utcCurrent = m_utcCurrent.addYears(years);
+    return m_utcCurrent;
+}
+
+void TestClock::setup(Clock* clock)
+{
+    Clock::setInstance(clock);
+}
+
+void TestClock::teardown()
+{
+    Clock::resetInstance();
+}
+
+QDateTime TestClock::currentDateTimeUtcImpl() const
+{
+    return m_utcCurrent;
+}
+
+QDateTime TestClock::currentDateTimeImpl() const
+{
+    return m_utcCurrent.toLocalTime();
+}