Fixing crash on linux. Exporting png in the UI thread.

xs/src/slic3r/AppController.hpp

@@ -147,6 +147,8 @@ public:
      */
     bool supports_asynch() const;
 
+    void process_events();
+
 protected:
 
     /**
@@ -200,6 +202,8 @@ protected:
     // The previous export data, to pre-populate the dialog
     PngExportData prev_expdata_;
 
+    void slice(ProgresIndicatorPtr pri);
+
 public:
 
     // Must be public for perl to use it