Popup widget position recalculated if parent widget resized

The ctkPopupWidget resides in its own window that should follow the
application window while it is being moved or resized. (This matters
only if the pop-up widget is pinned to the application window,
otherwise it is not visible during moving or resizing.)

This commit corrects the calculation of the new window position.

Fixes #411.
Merge branch '411-ctkPopupWidget-put-in-wrong-position-on-resize-or-move' of https://github.com/NifTK/CTK into NifTK-411-ctkPopupWidget-put-in-wrong-position-on-resize-or-move

Conflicts:
	Libs/Widgets/ctkPopupWidget.cpp