Implement export menu/button

[libreliodev]

As a reminder, the format used by our viewers is native pdf, so we need the plug-in to generate a web optimized pdf.

We also need the plug-in to gather all local assets (videos, photos) used by the file in one single folder. In order to do so:

• create a directory with the same name as the export file
• copy all assets used into this directory

[libreliodev]

As a first step, we will not export any asset. This will be done in later issues. So, to complete this issue, we just want to create a directory (open a dialog box for the user to specify where) and generate the web optimized pdf. Please not that the pdf should not contain overlays (this will be handled in other issues), but should contain all links added to the file.