Directional runs of text are currently ordered LTR. This is fine for unidirectional text (because the whole text consists of a single directional run) and for LTR text with embedded RTL, but we also need to support RTL text with embedded LTR, such as Arabic text containing Latin terms.

Allow setting the primary paragraph direction to RTL, so that embedded runs of LTR text do not break the overall direction of text.

This is a prerequisite for #6.