Following up from PR #4006. There is an additional RTL property available. It controls the placement of bidirectional neutral characters (mainly punctuation), as opposed to strong (alphabetic characters) or weak (numeric characters), especially at the beginning or end of a line. The new Comment property textboxDirection will be used for that purpose.
In a discussion in issue #4004 following the implementation of the PR, the comment was mixed RTL and LTR, and this led to some formatting problems. The user was able to overcome these with the timely insertion of Unicode directional control characters, but it would be preferable to have it happen automatically, which this change will permit. However, the use of these control characters cannot be entirely done away with. In the new test case, if one of the all-English lines ended with, say, a colon, it would not display correctly; LRM (left-to-right mark) after the colon would be needed. Likewise, one or two of the comment lines with mixed RTL and LTR (discussed in the issue) is not formatted correctly, and might require LRO/PDF or equivalent.
Fix#4004. RTL text can be included in a comment, and will display correctly, but the comment as a whole will be left-aligned. There already exists an `alignment` property for Comment, but it is unused. This PR will allow that property to be set, and to be read from and written to an Xlsx spreadsheet when possible.
The Xml tags that govern this property are found, unusually, in a drawing Vml file. The important property is `x:TextHAlign`, which can be set to Left, Right, Center, Justified, or Distributed. There are other tags which seemed like they were relevant to this problem, but I don't think they actually matter:
```xml
<v:textbox style='mso-direction-alt:auto'>
<div style='text-align:right;direction:rtl'></div>
</v:textbox>
```
oleibman