LinGui: make sure "Plot" tag gets updated before starting encoding

GTK is *really* stupid sometimes.  When "Start" is pressed, the button
event is activated *before* the focus-out event for the current in-focus 
widget.  So any final changes to that widget are not acted upon till after
processing the "Start" button :-\


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6575 b64f7644-9d1e-0410-96f1-a4d463321fa5