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/PrintConfig.hpp

@@ -619,6 +619,7 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionBool,                support_on_build_plate_only))
     ((ConfigOptionBool,                support_critical_regions_only))
     ((ConfigOptionFloat,               support_top_z_distance))
+    ((ConfigOptionFloat,               support_bottom_z_distance))
     ((ConfigOptionInt,                 enforce_support_layers))
     ((ConfigOptionInt,                 support_filament))
     ((ConfigOptionFloat,               support_line_width))