send newline char when input type is multiline

[maks]

Description

This PR re-instates the functionality for sending a newline char for TextFields on Linux desktop which have a keyboardType: TextInputType.multiline set on the widget. This functionality was lost in the move from the GLFW to the new GTK embedding.

Related Issues

Fixes: flutter/flutter#62207

Tests

There is no existing test for this, so the regression was not picked up in the move from glfw to gtk. I'm sorry but I don't know how to go about adding a test for this.

Checklist

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Breaking Change

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

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[domesticmouse]

/cc @stuartmorgan

[maks]

@stuartmorgan thanks for the very quick review! Hopefully I've fixed all the issues from your feedback.

[stuartmorgan]

Functionality looks great, just minor comment nits and then it can be landed. Thanks!

[stuartmorgan]

LGTM! I'll set the CQ bit once tests have come back green. Thanks for fixing this!