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/QtCodeFile.h

@@ -47,6 +47,7 @@ public:
 	bool isCollapsed() const;
 
 	void requestContent();
+	void requestWholeFileContent();
 	void updateContent();
 
 	void setWholeFile(bool isWholeFile, int refCount);