Partial inline widget support

[matthew-carroll]

Support a subset of inline widget use-cases:

• Auto-replace keywords, like :fire:, with a small bitmap (non-emoji). The bitmap can appear anywhere in the text, and therefore it needs to play nice with text selection.
• Show progress at the end of a text blob, e.g., "✅ 0/2". This widget should not be selectable at all.

The reason that we're supporting partial inline widgets, instead of full-featured inline widgets, is because Flutter's text layout algorithm can't break a widget across multiple lines, which is required for use-cases like inline code.

Funding

Superlist, Turtle

[bk-one]

This is a requirement for Superlist and I'd like to see this latest directly after 0.3.