Limit the size of explainPopup

Hi,

I often use pgadmin's graphical explain to see how queries generated
be hibernate are planned&executed by postgresql, and it works really
well.
However, for very long queries pgadmin3 tries to create windows >
65536 pixels, which is not supported by X11 and therefor leads to
XErrors, which in turn cause pgadmin3 to close because of assetions.

I would propose to limit the size of the ExplainPopup to the screen
dimensions, please see the attached patch.

Thank you in advance, Clemens