RTL bug in channel-and-topic label span

[chrisbobbe]

With #1877, we'll have a nice reusable function to make a channel-and-topic text span, like so:

And #1906 is for using it in more places.

Unfortunately, it's broken in RTL languages; the channel icon and the chevron icon are shown out of order with each other¹:

The symptom is a match for this issue in the Flutter tracker:

• WidgetSpans in RTL text laying out LTR if they occur on the same line flutter/flutter#54400

Footnotes

1. There's also fact that the "chevron-right" icon should actually face left in RTL. Let's track that separately, in RTL: Flip directional icons, like chevron_right and send, in RTL mode #1907. ↩