PartDesign: Refactor single-solid rule enforcement

This refactors a single solid rule checking code from using the solid
count directly to using well abstracted `isSingleSolidRuleSatisfied`
method. This makes code easier to read and is the basis for next step
which is allowing users to disable this checks.

src/Mod/PartDesign/App/FeatureBoolean.cpp

@@ -153,8 +153,7 @@ App::DocumentObjectExecReturn *Boolean::execute()
 
     result = refineShapeIfActive(result);
 
-    int solidCount = countSolids(result);
-    if (solidCount > 1) {
+    if (!isSingleSolidRuleSatisfied(result)) {
         return new App::DocumentObjectExecReturn(QT_TRANSLATE_NOOP("Exception", "Result has multiple solids: that is not currently supported."));
     }