only open a `layer` if the content to write is not empty

[Doublonmousse]

For #985

Changing how layer elements are created to circumvent bugs that pushed all images down into the background instead of being editable in xournal.

I wonder if I should add to this PR some other modifications to export more shapes to their xournal equivalent.

I've glanced over the xournal code for line and rectangle as well as how these things are exported in the xournal file and it seems feasible to convert them to their xournal equivalent (in their non rough version).