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/MacOpenGLWindow.mm

@@ -989,8 +989,48 @@ void MacOpenGLWindow::setRequestExit()
 	m_internalData->m_exitRequested = true;
 }
 
+#include <string.h>
+int MacOpenGLWindow::fileOpenDialog(char* filename, int maxNameLength)
+{
+    //save/restore the OpenGL context, NSOpenPanel can mess it up
+    //http://stackoverflow.com/questions/13987148/nsopenpanel-breaks-my-sdl-opengl-app
+    
+    NSOpenGLContext *foo = [NSOpenGLContext currentContext];
+    // get the url of a .txt file
+    NSOpenPanel * zOpenPanel = [NSOpenPanel openPanel];
+    NSArray * zAryOfExtensions = [NSArray arrayWithObject:@"urdf"];
+    [zOpenPanel setAllowedFileTypes:zAryOfExtensions];
+    NSInteger zIntResult = [zOpenPanel runModal];
+    
+    [foo makeCurrentContext];
+    
+    if (zIntResult == NSFileHandlingPanelCancelButton) {
+        NSLog(@"readUsingOpenPanel cancelled");
+        return 0;
+    }
+    NSURL *zUrl = [zOpenPanel URL];
+   if (zUrl)
+   {
+       //without the file://
+       NSString *myString = [zUrl absoluteString];
+       int slen = [myString length];
+       if (slen < maxNameLength)
+       {
+           const char *cfilename=[myString UTF8String];
+           //expect file:// at start of URL
+           const char* p = strstr(cfilename, "file://");
+            if (p==cfilename)
+            {
+                int actualLen = strlen(cfilename)-7;
+                memcpy(filename, cfilename+7,actualLen);
+                filename[actualLen]=0;
+                return actualLen;
+            }
+       }
+   }
 
-
+    return 0;
+}