Make the Qt 4 style signal-slots usages easy to find across subprojects

The search result currently only lists the usages for "find usage" in the current subproject. It would be nice to have a feature to find the usages in all subprojects. The current workaround is to either migrate to Qt 5 style or search in "All Projects". The former can be a decent amount of work, while the later works OK, but still.