Added a new placeholder for custom toolchange gcode (toolchange_z)

This always contains the actual print_z of the toolchange, while layer_z contains the print_z
of the print. The two differ in case that wipe tower without sparse layers is used.

Related to #3834.

src/libslic3r/GCode.cpp

@@ -285,6 +285,7 @@ namespace Slic3r {
             config.set_key_value("next_extruder", new ConfigOptionInt((int)new_extruder_id));
             config.set_key_value("layer_num", new ConfigOptionInt(gcodegen.m_layer_index));
             config.set_key_value("layer_z", new ConfigOptionFloat(tcr.print_z));
+            config.set_key_value("toolchange_z", new ConfigOptionFloat(z));
 //            config.set_key_value("max_layer_z", new ConfigOptionFloat(m_max_layer_z));
             toolchange_gcode_str = gcodegen.placeholder_parser_process("toolchange_gcode", toolchange_gcode, new_extruder_id, &config);
             check_add_eol(toolchange_gcode_str);
@@ -3003,6 +3004,7 @@ std::string GCode::set_extruder(unsigned int extruder_id, double print_z)
         config.set_key_value("next_extruder",     new ConfigOptionInt((int)extruder_id));
         config.set_key_value("layer_num",         new ConfigOptionInt(m_layer_index));
         config.set_key_value("layer_z",           new ConfigOptionFloat(print_z));
+        config.set_key_value("toolchange_z",      new ConfigOptionFloat(print_z));
         config.set_key_value("max_layer_z",       new ConfigOptionFloat(m_max_layer_z));
         toolchange_gcode_parsed = placeholder_parser_process("toolchange_gcode", toolchange_gcode, extruder_id, &config);
         gcode += toolchange_gcode_parsed;

src/libslic3r/PrintConfig.cpp

@@ -2438,7 +2438,7 @@ void PrintConfigDef::init_fff_params()
     def = this->add("toolchange_gcode", coString);
     def->label = L("Tool change G-code");
     def->tooltip = L("This custom code is inserted before every toolchange. Placeholder variables for all PrusaSlicer settings "
-                     "as well as {previous_extruder} and {next_extruder} can be used. When a tool-changing command "
+                     "as well as {toolchange_z}, {previous_extruder} and {next_extruder} can be used. When a tool-changing command "
                      "which changes to the correct extruder is included (such as T{next_extruder}), PrusaSlicer will emit no other such command. "
                      "It is therefore possible to script custom behaviour both before and after the toolchange.");
     def->multiline = true;