WDialogs do not resize smoothly beyond bounds of body
With github 3.3.3-25-g7023b38 version of Wt, attempting to resize a WDialog beyond outside the html body can be difficult. If the mouse moves too quickly, the dialog stops receiving the mousemove events and the drag is interrupted. This can be seen with the attached test program by using the button to create some WDialogs and then attempting to resize them to a point below the grid.
The attached patch attempts to address this issue by placing the mousemove and mouseup event handlers on the document. It seems to work in limited testing and does not appear to interfere with existing handlers, but the patch would definitely benefit from additional review.
This is the last in a series of WDialog-related patches that were (lightly) tested together, but should work separately.