ENH: add initial layer time to plate_x.json and slice_info.config

Jira: STUDIO-14504
Change-Id: I32460f1c19cce7c729806e35370e8b4b57aa069e
(cherry picked from commit 04a487d210134c28245c1f0f7d0986266a1c66f2)

src/libslic3r/GCode.hpp

@@ -618,6 +618,8 @@ private:
 
     std::vector<const PrintObject*> m_printed_objects;
 
+    float m_initial_layer_time;
+
     // Processor
     GCodeProcessor m_processor;