Editor popup will now reflect the direction of the opened code block.
This also updates in-editor codemirror instances to correcly reflect/use
the direction if set on the inner code elem.
This also defaults new code blocks, when in RTL languages, to be started
in LTR, which can then be changed via in-editor direction controls if
needed. This is on the assumption that most code will be LTR (could not
find much examples of RTL code use).
Fixes#4943
f9e087330b