Add command to generate image of a UI session directly from a .gfr script

[bentoogood]

To automate generation of images for the documentation, it would be nice to be able to call a command from the Gaffer api that reads a .gfr script and generates a 'screen capture' of the UI.
Should avoid actually launching the gaffer UI application if possible.
Some flags would be useful:

• whether to load the UI layout from the .gfr script or not (default to loading from script)
• whether to grab the entire UI or just the contents of a single panel or tab (default to entire UI)

how would be handle layouts with floating panels?

[bentoogood]

possibly using QtGui.QPixmap.grabWindow/Widget?

example for maya:

from PyQt4 import QtCore, QtGui
import sip
import maya.cmds as cmds
import maya.OpenMayaUI as mui

def getMayaWindow():
    ptr = mui.MQtUtil.mainWindow()
    return sip.wrapinstance(long(ptr), QtCore.QObject)

pm = QtGui.QPixmap.grabWindow(getMayaWindow().winId())
pm.save('/tmp/test.png')

[bentoogood]

this worked for me in gaffer:

from PyQt4 import QtCore, QtGui
pm = QtGui.QPixmap.grabWindow( QtGui.QApplication.activeWindow().winId() )
pm.save('/tmp/foo.png')

[bentoogood]

Addressed by screengrab app #350 #336

Plan is to save gui state as per #306 - saving individual panels maximised if only one editor is required (rather than adding functionality to grab specific widgets).