v0.31.0

• TIFY is now available in Croation (thanks to @hrvoj3e) and Italian (thanks to @atomotic).
• The table of contents has been improved to better handle some edge cases.
• A missing translation no longer breaks the app. If a translation cannot be loaded, an error message is displayed and the previous language is kept.
• The ready promise is now fulfilled only after viewer is initialized, allowing to use the OpenSeadragon API as documented.