Draft: Auto select current document in Export Layers

The Export Layers dialog annoyingly doesn't auto select the currently active document, meaning that if you run the script and just mash the OK button you'll get a dialog telling you nothing is selected. This improves the workflow by auto-selecting whichever document is currently the active one.

Also improved the UI of the list box to only show the document's base name, rather than the full path. The full path is available to the user as a tool tip.

Added multi-document exporting, so users can select multiple items in the document listbox and batch export them all in one go.