GTextEditor: Add a way to flush any pending on_change notifications

Since on_change handlers can alter the text document we're working on,
we have to make sure they've been run before we try looking at spans.
This fixes some flakiness when a paint happened before HackStudio had
a chance to re-highlight some C++ while editing it.

The design where clients of GTextEditor perform syntax highlighting in
the "arbitrary code execution" on_change callback is not very good.
We should find a way to move highlighting closer to the editor.

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