skins2: resize only similar layouts on-the-fly

skins2: resize only similar layouts on-the-fly

This fixes a regression brought by ff74e967e9777ed821734077051f55ac84a173a5

Forcefully resizing only makes sense when layouts were designed to be similar
in size in the first place. Furthermore, it cuts off a larger layout if a
smaller one is first defined when loading a skin.

Ideally, the skins2 dtd should be extended for skins developers to explicitly
express whether two layouts are tightly-coupled or not, i.e should always be
kept the same size whatever the subsequent resizing performed by the user.