Sketcher: change rules of external linkage

Was:
+ allowed: links to support
+ allowed: free links within one body,
+ Ctrl-protected: links between bodies of ?one? part
- blocked: (everything else) . i.e., if sketch is not in a body, only
links to support are allowed.

New:

* Across Parts, from Part to outside part, from document into part:
blocked
* Within one part (also if directly in document): allowed.
except links from body to outside or to other bodies are screened (hold
Ctrl to allow).
Support (attachment) is totally ignored in this logic.

src/Mod/Sketcher/App/SketchObject.cpp

@@ -58,6 +58,7 @@
 
 #include <App/Document.h>
 #include <App/FeaturePythonPyImp.h>
+#include <App/Part.h>
 #include <Base/Writer.h>
 #include <Base/Reader.h>
 #include <Base/Tools.h>
@@ -1729,20 +1730,32 @@ bool SketchObject::isExternalAllowed(App::Document *pDoc, App::DocumentObject *p
 
     // Note: Checking for the body of the support doesn't work when the support are the three base planes
     App::DocumentObject *support = this->Support.getValue();
-    Part::BodyBase* body = Part::BodyBase::findBodyOf(this);
-    if (body != NULL) {
-        if ( ! body->hasFeature (pObj) && !this->allowOtherBody ) {
-            // Selection outside of body not allowed if flag is not set
-            if (rsn)
-                *rsn = rlOtherPart;
-            return false;
+    Part::BodyBase* body_this = Part::BodyBase::findBodyOf(this);
+    Part::BodyBase* body_obj = Part::BodyBase::findBodyOf(pObj);
+    App::Part* part_this = App::Part::getPartOfObject(this, true);
+    App::Part* part_obj = App::Part::getPartOfObject(pObj, true);
+    if (part_this == part_obj){ //either in the same part, or in the root of document
+        if (body_this == NULL) {
+            return true;
+        } else if (body_this == body_obj) {
+            return true;
+        } else {
+            if ( this->allowOtherBody ) { // Selection outside of body not allowed if flag is not set
+                return true;
+            } else {
+                if (rsn)
+                    *rsn = rlOtherBody;
+                return false;
+            }
         }
     } else {
-        // Legacy parts - don't allow selection outside of the support
-        if (pObj != support)
-            return false;
+        // cross-part link. Disallow, should be done via shapebinders only
+        if (rsn)
+            *rsn = rlOtherPart;
+        return false;
     }
 
+    assert(0);
     return true;
 }