Offer a way to select the pages to be imported

[michitux]

It would be nice if the space selection screen could also display the pages to be imported as a tree and allow selecting which pages should be imported. That tree should only be loaded on demand. If it should turn out to be too difficult/costly to load the tree on demand, the page selection could also be enabled by an option similar to the "skip space selection" option.

This would in particular be helpful if a full import has already been performed and just a few pages shall be imported again.