Ft.io abstractions

This is a whole revamp on how Apostrophe handles files:

• It migrates the internal machinery to use GFile
• Better handling of file opening (from Apostrophe/file manager/terminal)
• It migrates from Gtk.FileChooser to Gtk.FileChooserNative
• Better sandboxing: Apostrophe (mostly) will work fine even without any kind of filesystem access
• Use of async I/O operations when possible
• Adds feedback for file saving in the form of an animated progressbar
• Adds a new Advanced Export dialog
• It reworks the entirety of the file exporting code
• Adds the necessary assets for reveal.js slideshows on export
• Migrates the pdf engine to xelatex