List Block: Increase/Decrease Indent icons should be switched when using RTL language.

[kjellr]

Describe the bug

When using a LTR language, the arrows on the increase/decrease indent buttons on the List block indicate the direction the text will move when you press the button:

When you switch to a RTL language, the arrows do not point in the direction that the text will move:

I'd argue that these icons should be swapped when a RTL language is being used, so that the arrows properly represent the direction text will be moving.

To Reproduce

1. Switch to a RTL language (Arabic, Hebrew, etc)
2. Insert a list block with at least two items.
3. Toggle the indent, using the toolbar buttons.

Tested in

Mac OS 10.13.6, Safari 11.1.2
Mac OS 10.13.6, Chrome 67.0.3396.99

[draganescu]

As an example of prior art, Google Docs has slightly different icons:

• English

• Arabic

[Mamaduka]

This should be fixed after recent refactoring #39773.