Export treenodes: add basic front-end infrastructure
Treenode/connector export: make dialog usable for both exports
Treenode/connector export: rename source module to make more code reusable

Both, treenode and connector export, will share most of the code. Because
connectors can be considered as a special kind of treenodes, it makes sense to
have most code in a module referring to treenodes.
Treenode/connector export: refactor to support both export types
Treenode exporter: implement missing exporter functions