add fileOpenDialog and enable loading of urdf from GUI

(will add .bullet file support soon)
Uses native Windows (getFileOpenFileName) and Mac OSX NSOpenPanel,
on Linux using pipe popen to zenity)

btgui/OpenGLWindow/X11OpenGLWindow.cpp

@@ -959,4 +959,29 @@ b3KeyboardCallback      X11OpenGLWindow::getKeyboardCallback()
 {
 	return m_data->m_keyboardCallback;
 }
+#include <stdio.h>
 
+int X11OpenGLWindow::fileOpenDialog(char* filename, int maxNameLength)
+{
+	int len = 0;
+	FILE * output = popen("zenity --file-selection --file-filter=\"*.urdf\" --file-filter=\"*.*\"","r");
+	if (output)
+	{
+		while( fgets(filename, maxNameLength-1, output) != NULL )
+		{
+			len=strlen(filename);
+			if (len>0)
+			{
+				filename[len-1]=0;
+				printf("file open (length=%d) = %s\n", len,filename);
+			}	
+		}
+		pclose(output);
+	} else
+	{
+		printf("Error: fileOpenDialog no popen output, perhaps install zenity?\n");
+	}
+	XRaiseWindow(m_data->m_dpy, m_data->m_win);
+	return len;
+	
+}