Make the Linux GTK widget focusable

[jason-simmons]

Handling of keyboard focus for Flutter text fields appears to have regressed with #39473, which removed the call to gtk_widget_set_can_focus

[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 (don't just cc him here, he won't see it! He's on Discord!).

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[cbracken]

This lgtm. Thanks for spotting. I'm guessing we didn't have a test for this or it would already have been failing.

It looks like we should be able to test this with gtk_widget_get_can_focus.

[robert-ancell]

There were a lot of focus related calls that seemed redundant in the code before the simplification, I guess I took out one too many. Confirmed it was broken and this fixes it.

[robert-ancell]

Adding a test for gtk_widget_get_can_focus would catch a regression in this one particular line, but what we really want is a test that generates a key event and confirms it gets all the way to Flutter. There's a lot of focus / keyboard handling code that could regress.

Hopefully this PR can get a test exemption, but the test proposed by @cbracken should be sufficient if not.