ENH: popup different warning message for unsupported sharp tails

1. unsupported sharp tails and large overhang have different messages
2. do not popup warning for total area (only warning for one single
   large overhang)

Change-Id: I153e559915a92c6676468539cf619dafe915f997
(cherry picked from commit 67b593150f8de847d9b1999bf0d7a549eb1ef154)

src/libslic3r/Print.hpp

@@ -49,6 +49,12 @@ struct groupedVolumeSlices
     ExPolygons              slices;
 };
 
+enum SupportNecessaryType {
+    NoNeedSupp=0,
+    SharpTail,
+    LargeOverhang,
+};
+
 namespace FillAdaptive {
     struct Octree;
     struct OctreeDeleter;
@@ -457,7 +463,7 @@ private:
     std::pair<FillAdaptive::OctreePtr, FillAdaptive::OctreePtr> prepare_adaptive_infill_data();
 
     // BBS
-    bool is_support_necessary();
+    SupportNecessaryType is_support_necessary();
 
     // XYZ in scaled coordinates
     Vec3crd									m_size;