3.0-RC2

• Renamed "filters" to "constraints" to avoid confusion with the concept of
  image filters.
• Adding a previously enabled and subsequently removed operation or constraint
  will now be disabled when added back again.
• Moved pygimplib folder above the export_layers folder.
• Changed user documentation to HTML (more-or-less a copy of the plug-in page).
• Fixed operations being ignored for "Export Layers (repeat)".
• Fixed dialog position resetting after closing the file format dialog.
• Fixed tagged layers (notably foreground and background) not working for
  indexed images.
• Added French, Russian and Ukrainian translations.