Add template based Document::countObjectsOfType

Also convert code to use this new method

src/Mod/Sketcher/Gui/Command.cpp

@@ -763,12 +763,8 @@ void CmdSketcherMapSketch::activated(int iMsg)
 bool CmdSketcherMapSketch::isActive()
 {
     App::Document* doc = App::GetApplication().getActiveDocument();
-    Base::Type sketch_type = Base::Type::fromName("Part::Part2DObject");
     std::vector<Gui::SelectionObject> selobjs = Gui::Selection().getSelectionEx();
-    if (doc && doc->countObjectsOfType(sketch_type) > 0 && !selobjs.empty())
-        return true;
-
-    return false;
+    return doc && doc->countObjectsOfType<Part::Part2DObject>() > 0 && !selobjs.empty();
 }
 
 DEF_STD_CMD_A(CmdSketcherViewSketch)