Support right-to-left languages

[osma]

To properly support Arabic (#767) and other RTL languages, we need changes in the UI. In my understanding, there needs to be a dir=rtl setting in HTML but also the CSS styles need some adaptation because of settings that affect right vs. left.

[osma]

See also this discussion on skosmos-users, which contains some hints on how to implement this.

[osma]

My basic idea (perhaps overly idealistic) would be this:

• Change the HTML template and PHP code so that they add the dir="rtl" attribute when the UI locale is an RTL language. For consistency, dir="ltr" could be used in the other cases.
• Separate out the CSS rules which are affected by ltr vs. rtl, e.g. margin-right/margin-left and similar where the direction has to be hardcoded, into their own blocks. Make these CSS rules dependent on the dir="ltr" vs dir="rtl" attribute, so that only the rules for the right direction are applied.
• There should be no need to change the other templates, but I may be wrong.
• For JavaScript code, similar distinction may be needed. These should also look at the dir attribute and change their behavior based on that.

Here are tutorials I found earlier:

https://hacks.mozilla.org/2015/09/building-rtl-aware-web-apps-and-websites-part-1/

http://blog.scottlogic.com/2015/02/13/rtl.html

[osma]

Here's how to find out if a particular locale is LTR or RTL:
https://stackoverflow.com/questions/47118326/how-to-get-direction-of-a-language-by-its-local-code-rtl-or-ltr