This commit permits language plugins to carry the field
“text-direction” with the value “rtl” to trigger right-to-left layout
of the entire page. We also adjust the sidebar layout to work in RTL
mode.
There are still a number of problems to be addressed:
* Brackets and other punctuation incorrectly placed within en-GB UI text
* System tiddler titles are rendered semi-back-to-front (eg
`languages/ca-ES/:$`)
Starting to address #1845 and the discussion in #2523.
