Fix issue of lossing syntax highlighting during "save as"

The new behaviour is: as user set a new syntax highlighting once, the
new file extension of "save as" will be ignored.

Fixes #1298

PowerEditor/src/NppCommands.cpp

@@ -2789,6 +2789,10 @@ void Notepad_plus::command(int id)
 		case IDM_LANG_USER :
 		{
             setLanguage(menuID2LangType(id));
+			// Manually set language, don't change language even file extension changes.
+			Buffer *buffer = _pEditView->getCurrentBuffer();
+			buffer->langHasBeenSetFromMenu();
+
 			if (_pDocMap)
 			{
 				_pDocMap->setSyntaxHiliting();

PowerEditor/src/ScitillaComponent/Buffer.cpp

@@ -205,7 +205,8 @@ void Buffer::setFileName(const TCHAR *fn, LangType defaultLang)
 	}
 
 	updateTimeStamp();
-	if (newLang != _lang || _lang == L_USER)
+
+	if (!_hasLangBeenSetFromMenu && (newLang != _lang || _lang == L_USER))
 	{
 		_lang = newLang;
 		doNotify(BufferChangeFilename | BufferChangeLanguage | BufferChangeTimestamp);

PowerEditor/src/ScitillaComponent/Buffer.h

@@ -351,6 +351,8 @@ public:
 	void setMapPosition(const MapPosition & mapPosition) { _mapPosition = mapPosition; };
 	MapPosition getMapPosition() const { return _mapPosition; };
 
+	void langHasBeenSetFromMenu() { _hasLangBeenSetFromMenu = true; };
+
 private:
 	int indexOfReference(const ScintillaEditView * identifier) const;
 
@@ -408,5 +410,7 @@ private:
 	HANDLE _eventHandle = nullptr;
 	bool _isMonitoringOn = false;
 
+	bool _hasLangBeenSetFromMenu = false;
+
 	MapPosition _mapPosition;
 };