Utils: Remove FilePath::operator+()

This was a alias for .stringAppended(), but can be used too easily
when .pathAppended() is meant.

Change-Id: Ia3b64d39828d4074b43d87c923ce3a6a87038948
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/qmlprojectmanager/qmlproject.cpp

@@ -125,7 +125,7 @@ QmlProject::QmlProject(const Utils::FilePath &fileName)
                                   });
                               } else {
                                   Utils::FilePaths uiFiles = getUiQmlFilesForFolder(projectDirectory()
-                                                                                    + "/content");
+                                                                                    / "content");
                                   if (uiFiles.isEmpty())
                                       uiFiles = getUiQmlFilesForFolder(projectDirectory());