Fixed a regression error: The "current_extruder" identifier was not set

at the placeholder parser.
Implemented a new PlaceholderParser::evaluate_boolean_expression()
functionality to evaluate just a boolean expression using the full
expressive power of the macro processing syntax. This function
will now be used for deciding, which print or filament preset
is compatible with which printer preset.

xs/xsp/PlaceholderParser.xsp

@@ -21,4 +21,13 @@
                 croak(e.what());
             }
         %};
+
+    bool evaluate_boolean_expression(const char *str) const
+        %code%{
+            try {
+                RETVAL = THIS->evaluate_boolean_expression(str, THIS->config());
+            } catch (std::exception& e) {
+                croak(e.what());
+            }
+        %};        
 };