Everywhere: Use MonotonicTime instead of Duration

This is easily identifiable by anyone who uses Duration::now_monotonic,
and any downstream users of that data.

Userland/Libraries/LibGUI/UndoStack.cpp

@@ -81,7 +81,7 @@ void UndoStack::set_current_unmodified()
         return;
 
     m_clean_index = m_stack_index;
-    m_last_unmodified_timestamp = Duration::now_monotonic();
+    m_last_unmodified_timestamp = MonotonicTime::now();
 
     if (on_state_change)
         on_state_change();