NEW: Open "Bottom Z distance" setting for users

Users can set distance between support bottom contacts and object
once enable normal/tree support.

Jira: STUDIO-1383

TODO：Func seems good though, some details (e.g., bottom_interface
detection) need to be optimized furthur. Will keep follow-up.

Change-Id: I85815e7aa6cf1a5d0249633cd8ab995873461e8a
(cherry picked from commit eab39e89c94d749ebe6266ab745cbb70a068277d)

src/libslic3r/PrintObject.cpp

@@ -707,6 +707,7 @@ bool PrintObject::invalidate_state_by_config_options(
 		} else if (
                opt_key == "elefant_foot_compensation"
             || opt_key == "support_top_z_distance"
+            || opt_key == "support_bottom_z_distance"
             || opt_key == "xy_hole_compensation"
             || opt_key == "xy_contour_compensation") {
             steps.emplace_back(posSlice);