Layout on Wx when groups not visible

Fixed in Qt as part of #56, but still an issue in Wx:
- the size of a dialog that contains elements that are activated by "visible_when" ended up being the sum of the sizes of the elements, even though the elements are mutually exclusive (a typical case is a dropbox that lets you select different cases that need a different set of parameters). The expected behavior is that the size of the dialog should be at most the size of the largest combination of elements.