ui: file not up to date display

* the codeview now displays a hatched background for a filename when the file's content has changed or the file does not exist anymore.
* added TimePoint class to encapsulate boost.

src/lib/data/access/StorageAccessProxy.cpp

@@ -6,6 +6,7 @@
 
 #include "utility/logging/logging.h"
 #include "utility/file/FileInfo.h"
+#include "utility/TimePoint.h"
 
 StorageAccessProxy::StorageAccessProxy()
 	: m_subject(nullptr)
@@ -243,3 +244,13 @@ std::shared_ptr<TextAccess> StorageAccessProxy::getFileContent(const FilePath& f
 
 	return nullptr;
 }
+
+TimePoint StorageAccessProxy::getFileModificationTime(const FilePath& filePath) const
+{
+	if (hasSubject())
+	{
+		return m_subject->getFileModificationTime(filePath);
+	}
+
+	return TimePoint(boost::posix_time::not_a_date_time);
+}