Utils: Introduce FilePath::operator/(const QString &)

Including some random uses.

This mimics  std::filesystem::operator/(std::filesystem::path).

Change-Id: I0b0f5cf0d962fd33d4cbb9be96645a0b4a21ee03
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/git/gerrit/gerritparameters.cpp

@@ -73,7 +73,7 @@ static inline QString detectApp(const char *defaultExe)
     const QStringList entries = dir.entryList({"mingw*"});
     if (entries.isEmpty())
         return QString();
-    path = path.pathAppended(entries.first()).pathAppended("bin").pathAppended(defaultApp);
+    path = path / entries.first() / "bin" / defaultApp;
     if (path.exists())
         return path.toString();
     return QString();