QmlProject: Add mainFile property

The mainFile property of QmlProject is the default file to run. People
have still the opportunity to override this in their run settings,
though.

The wizard generated code was updated accordingly. Note that this makes
projects generated by the wizard incompatible with QtCreator 2.1!

Task-number: QTCREATORBUG-3249

src/plugins/qmlprojectmanager/qmlprojectrunconfiguration.h

@@ -81,8 +81,15 @@ public:
     int qtVersionId() const;
     Qt4ProjectManager::QtVersion *qtVersion() const;
 
+    enum MainScriptSource {
+        FileInEditor,
+        FileInProjectFile,
+        FileInSettings
+    };
+    MainScriptSource mainScriptSource() const;
+    void setScriptSource(MainScriptSource source, const QString &settingsPath = QString());
+
     QString mainScript() const;
-    void setMainScript(const QString &scriptFile);
 
     Utils::Environment environment() const;