MWExtensionInspector input direction based on content

In general, the direction of the MWExtensionInspector textarea
should be dependent on the directionality of the node it is editing.

The only exceptions are <hiero> and <math> that need to have their
textarea LTR always; these two inspectors' directionality definition
is overridden in their onOpen() method.

Bug: 56779
Change-Id: Iac5c1c3bf2c61b9fa36c9588c1734c91ca4305c4
Merge "MWExtensionInspector input direction based on content"