Add backwards compatibility for legacy XML project files (data levels 3-5)

Reintroduce read-only support for the old XML-based .mbs project file
format. The XML parser converts legacy files into the same ProjectSettings
structure used by the JSON parser, including constructing the adapter
settings JSON blob from XML connection/device elements. Saving always
produces JSON v6, effectively auto-migrating old files.

All XML-specific code is isolated in separate files (ProjectFileXmlParser)
for easy future removal. The only change to existing code is a small
format-detection branch in ProjectFileHandler::openProjectFile().

https://claude.ai/code/session_01Lyu9U25MtH34zG7gUFLe2u

Author: claude

src/importexport/projectfiledefinitions.h

@@ -10,10 +10,26 @@ const char cModbusScopeTag[] = "modbusscope";
 const char cModbusTag[] = "modbus";
 const char cScopeTag[] = "scope";
 const char cViewTag[] = "view";
+const char cConnectionTag[] = "connection";
 const char cDeviceTag[] = "device";
 const char cDeviceIdTag[] = "deviceid";
 const char cDeviceNameTag[] = "name";
 const char cLogTag[] = "log";
+const char cConnectionIdTag[] = "connectionid";
+const char cConnectionEnabledTag[] = "enabled";
+const char cConnectionTypeTag[] = "type";
+const char cIpTag[] = "ip";
+const char cPortTag[] = "port";
+const char cPortNameTag[] = "portname";
+const char cBaudrateTag[] = "baudrate";
+const char cParityTag[] = "parity";
+const char cDataBitsTag[] = "databits";
+const char cStopBitsTag[] = "stopbits";
+const char cSlaveIdTag[] = "slaveid";
+const char cTimeoutTag[] = "timeout";
+const char cConsecutiveMaxTag[] = "consecutivemax";
+const char cInt32LittleEndianTag[] = "int32littleendian";
+const char cPersistentConnectionTag[] = "persistentconnection";
 const char cPollTimeTag[] = "polltime";
 const char cAbsoluteTimesTag[] = "absolutetimes";
 const char cLogToFileTag[] = "logtofile";
@@ -58,7 +74,9 @@ const char cAdapterSettingsKey[] = "settings";
 const char cAdapterIdKey[] = "adapterId";
 const char cAdapterKey[] = "adapter";
 const char cIdJsonKey[] = "id";
+const char cConnectionsJsonKey[] = "connections";
 const char cDevicesJsonKey[] = "devices";
+const char cConnectionTypeJsonKey[] = "connectiontype";
 
 /* JSON constant values */
 const quint32 cMinimumJsonVersion = 6;

src/importexport/projectfilehandler.cpp

@@ -4,6 +4,7 @@
 #include "importexport/projectfiledata.h"
 #include "importexport/projectfilejsonexporter.h"
 #include "importexport/projectfilejsonparser.h"
+#include "importexport/projectfilexmlparser.h"
 #include "models/device.h"
 #include "models/graphdatamodel.h"
 #include "models/guimodel.h"
@@ -35,8 +36,22 @@ void ProjectFileHandler::openProjectFile(QString projectFilePath)
         QTextStream in(&file);
         QString projectFileContents = in.readAll();
 
-        ProjectFileJsonParser jsonParser;
-        GeneralError parseErr = jsonParser.parseFile(projectFileContents, &loadedSettings);
+        GeneralError parseErr;
+        QString trimmed = projectFileContents.trimmed();
+        if (trimmed.startsWith('{'))
+        {
+            ProjectFileJsonParser jsonParser;
+            parseErr = jsonParser.parseFile(projectFileContents, &loadedSettings);
+        }
+        else if (trimmed.startsWith('<'))
+        {
+            ProjectFileXmlParser xmlParser;
+            parseErr = xmlParser.parseFile(projectFileContents, &loadedSettings);
+        }
+        else
+        {
+            parseErr.reportError(tr("The file is not a valid MBS project file."));
+        }
 
         if (parseErr.result())
         {