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/view/QtCodeView.h

@@ -46,6 +46,8 @@ public:
 	virtual void showContents();
 
 	virtual bool isInListMode() const;
+	virtual void setMode(bool listMode);
+
 	virtual bool hasSingleFileCached(const FilePath& filePath) const;
 
 private: