Factor out wxGenericProgressDialog::EnsureActiveEventLoopExists()

Make this function reusable in order to allow it to be called from the
native MSW wxProgressDialog implementation in the upcoming commit.

include/wx/generic/progdlgg.h

@@ -105,6 +105,9 @@ protected:
     // Converts seconds to HH:mm:ss format.
     static wxString GetFormattedTime(unsigned long timeInSec);
 
+    // Create a new event loop if there is no currently running one.
+    void EnsureActiveEventLoopExists();
+
     // callback for optional abort button
     void OnCancel(wxCommandEvent&);