FIX: bring back Tree Strong style

Tree Strong is too similar to Tree Slim and people don't like it.

Jira: STUDIO-2660
Github: #1579
Change-Id: I52d7e8e674f0fc332e4ab2813261ffbc4b4cfb7a
(cherry picked from commit 9eb209b4c1737d2f10b9b8bd78c9f876f3d32bce)

src/libslic3r/TreeSupport.hpp

@@ -407,6 +407,7 @@ private:
     coordf_t MAX_BRANCH_RADIUS = 10.0;
     coordf_t MIN_BRANCH_RADIUS = 0.5;
     float tree_support_branch_diameter_angle = 5.0;
+    bool  is_strong = false;
     bool  is_slim                            = false;
     bool  with_infill                        = false;