Right to left support

An Arabic-language experiment is presently in the "Arabic-sample" folder, showing the current state of supporting right-to-left languages.

What works:

• Playback and highlighting Though for some sections, the highlight doesn't move past the heading, though the audio plays through. Unsure if the book is like this or not.
• Right-to-left display of text and some widgets

What needs updating:

• Conversion script should add the right lang code to all documents (nav, index and all chapters, about)
• Next/prev phrase buttons go the wrong way
• Audio volume and other sliders go the wrong way
• TOC still opens on left even though hamburger button switched to right side of screen