[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/TaskExtrudeParameters.cpp

@@ -84,7 +84,7 @@ void TaskExtrudeParameters::setupDialog()
     int faceId = -1;
     if (obj && !subStrings.empty()) {
         upToFace = subStrings.front();
-        if (upToFace.substr(0,4) == "Face")
+        if (upToFace.compare(0, 4, "Face") == 0)
             faceId = std::atoi(&upToFace[4]);
     }