[PD] Use compare to compare strings instead of substr

Used when finding subelements of a feature. Many of the comparisons used to also
check for string lengths, but as far as I can tell they are not strictly
necessary (see https://www.cplusplus.com/reference/string/string/substr/) and
just `substr` can be used without them. However, `compare` explicitly is for
comparing, and does not make a new object that `substr` does.

src/Mod/PartDesign/Gui/ViewProviderShapeBinder.cpp

@@ -159,13 +159,13 @@ void ViewProviderShapeBinder::highlightReferences(const bool on, bool /*auxiliar
 
             for (std::string e : subs) {
                 // Note: stoi may throw, but it strictly shouldn't happen
-                if(e.substr(4) == "Edge") {
+                if(e.compare(0, 4, "Edge") == 0) {
                     int idx = std::stoi(e.substr(4)) - 1;
                     assert ( idx>=0 );
                     if ( idx < (ssize_t) lcolors.size() )
                         lcolors[idx] = App::Color(1.0,0.0,1.0); // magenta
                 }
-                else if(e.substr(4) == "Face")  {
+                else if(e.compare(0, 4, "Face") == 0)  {
                     int idx = std::stoi(e.substr(4)) - 1;
                     assert ( idx>=0 );
                     if ( idx < (ssize_t) fcolors.size() )