Improve the scroll area size in pqProxyWidgetDialog

The pqProxyWidgetDialog is populated by a pqProxyWidget which can take any
size. Having a simple default size isn't very robust as the dialog is
used at different places with different pqProxyWidget.
To find the dialog size, we display it offscreen so we can resize it after.

Change-Id: I86755cb1ee9c0f0287ed6eb9714c2b0129f81f63
Merge topic '14734-GoodDefaultSizeForpqProxyWidgetDialog' into next

9759aca0 Improve the scroll area size in pqProxyWidgetDialog