PartDesign: New features AdditiveHelix and SubtractiveHelix

These features, based on the code for the Pipe class, allow the user
to simply create a helical sweep within PartDesign workbench.

Sample application is threads, springs, coils, augers, etc.

Also, remove needless requirement for positive cone angle on helixes.

Thanks to @bitacovir for helping with the icons
Thanks to @chennes for review
Thanks to @vosk for review
Thanks to @wwmayer for review

Enforce that links stay within scope for ProfileBased features
This also ensures that the Body itself is not used for creating features within
the body, causing a "Graph not a DAG" error.

src/Mod/PartDesign/App/Body.h

@@ -118,7 +118,7 @@ public:
     PyObject *getPyObject(void) override;
 
     virtual std::vector<std::string> getSubObjects(int reason=0) const override;
-    virtual App::DocumentObject *getSubObject(const char *subname, 
+    virtual App::DocumentObject *getSubObject(const char *subname,
         PyObject **pyObj, Base::Matrix4D *pmat, bool transform, int depth) const override;
 
     void setShowTip(bool enable) {
@@ -137,6 +137,9 @@ public:
       */
     App::DocumentObject *getNextSolidFeature(App::DocumentObject* start = NULL);
 
+    // a body is solid if it has features that are solid according to member isSolidFeature.
+    bool isSolid(void);
+
 protected:
     virtual void onSettingDocument() override;