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.h

@@ -47,6 +47,7 @@ public:
 	virtual std::shared_ptr<TokenLocationFile> getTokenLocationOfParentScope(const TokenLocation* child) const;
 
 	virtual std::shared_ptr<TextAccess> getFileContent(const FilePath& filePath) const;
+	virtual TimePoint getFileModificationTime(const FilePath& filePath) const;
 
 private:
 	StorageAccess* m_subject;