preload guest frame

Right now, caja.js does
  load taming frame
  when taming frame ready
    load guest frame
    when guest frame ready
      run gadget

This is unnecessarily serial.  Since we always need at least one guest
frame, we can start loading one before the taming frame is ready.

This change wraps guest frame creation in a frameMaker object that
can preload a frame and supply it to a later call to frameMaker.make().

[metaweta, 2011-12-13 23:28:09]

lgtm

[felix8a, 2011-12-13 23:58:39]

@r4681