Meta boxes arrow icons in RTL

[afercia]

Many things still need to fully support RTL languages in the UI. One small detail is about the meta boxes icons.

They're currently using a right-arrow icon when collapsed and a down-arrow icon when expanded:

See also related discussion on Slack https://wordpress.slack.com/archives/C02QB2JS7/p1498468801241441?thread_ts=1498209002.818056&cid=C02QB2JS7

If the right-arrow icon is going to stay, I guess it should be swapped to left-icon for RTL languages. I think this is something WordPress does during the build process with a custom grunt plugin swap-dashicons-left-right-arrows. Instead, Gutenberg sets these icons in the component, depending on the meta box opened state:
icon={ this.state.opened ? 'arrow-down' : 'arrow-right' }

/cc @ocean90

[jasmussen]

I built a fix for this one in #3831, which was then improved upon in #3844. As such, I'm closing this as fixed. Feel free to reopen if this was in error.