Improve resizing a box.

libgnt used to always change the size of the first widget. So in a lot of
dialogs (e.g. the log viewer, the add-buddy dialogs etc.) only the labels
would be resized, and the entry/comboboxes, resizing which would be more
useful, would never be resized. So change that behaviour to cycle through
all the widgets in the box for resize.

Fixes #11580, #7843.