ui: switch to single file mode with snippet maximize

* switch back to snippet view with snippet button in file title bar
* use new title bar widget in single view as well
* restore view mode on undo, store current view mode for every message id (added id to message)
* treat view mode switch as adapt message on undo stack
* fixed crash after snippet expansion
* fixed hatching in file title not shown

src/lib_gui/qt/element/QtCodeFileSingle.h

@@ -14,7 +14,7 @@
 class QLabel;
 class QPushButton;
 class QtCodeArea;
-class QtCodeFileTitleButton;
+class QtCodeFileTitleBar;
 class QtCodeNavigator;
 
 class QtCodeFileSingle
@@ -52,6 +52,9 @@ public:
 
 	Id getLocationIdOfFirstActiveLocationOfTokenId(Id tokenId) const;
 
+public slots:
+	void clickedSnippetButton();
+
 private:
 	struct FileData
 	{
@@ -73,8 +76,7 @@ private:
 	QWidget* m_areaWrapper;
 	FilePath m_currentFilePath;
 
-	QtCodeFileTitleButton* m_title;
-	QLabel* m_referenceCount;
+	QtCodeFileTitleBar* m_titleBar;
 
 	QtCodeArea* m_area;
 	std::map<FilePath, FileData> m_fileDatas;