FIX: empty layers and dangling hybrid support

1. fix possible empty layers (caused by continues walls)
2. fix normal node may be absorbed in drop_nodes in "tree hybrid"
3. popup message to ask the user to select best parameters for tree strong
   and tree hybrid.
4. auto select lightning infill for tree hybrid.

Change-Id: I7d172887a9b0e4f268267684e83fc02dc7ec6a0c
(cherry picked from commit bc6d130e86c252d7fc1f84c4df7da95050dd5d51)

src/libslic3r/TreeSupport.hpp

@@ -384,6 +384,7 @@ private:
     coordf_t MIN_BRANCH_RADIUS = 0.5;
     float tree_support_branch_diameter_angle = 5.0;
     bool  is_slim                            = false;
+    bool  with_infill                        = false;
 
 
     /*!