Prior to this change, a GUI layout was only affected by the screen size.
Now, a layout can additionally be influenced by the GUI dialog and widgets
that uses it. This capability is leveraged to implement the following
features:
* Layout elements that are not bound to a GUI widget do not take space.
This means that dialogs where the widgets shown depend on for example
a feature being enabled at configure time no longer have blank spaces.
* Widgets can define a minimal required size for their contents not to be
cut. For now this is only used for buttons so their width is always
sufficient for their caption not to be cut. This mechanism could be
applied to other widget types in the future.
