[file_selector] add GtkFileChooserDialog based implementation for Linux

[jpnurmi]

Description

This PR implements the FileSelector method channel interface for Linux. The implementation is based on GtkFileChooserDialog from GTK+ and has been tested with the example app in https://github.com/tugorez/plugins/tree/file_selector/packages/file_selector/file_selector/example

Related PRs

• [file_selector_platform_interface] Add platform interface for new file_selector plugin #2995
• [file_selector_web]: Add initial implementation #3141

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[jpnurmi]

I ran clang-format -i --style=Google file_selector_linux_plugin.cc to fix the format check.

[jpnurmi]

/cc @tugorez @jasonpanelli

[ditman]

Thanks for the contribution! I'm totally unqualified to review linux code for now; @stuartmorgan is the boss here!

[stuartmorgan-g]

@robert-ancell would be the best person to do the initial review here.

However, I'm confused why this was written from scratch when we already have the FDE file_chooser plugin; my plan was to simply convert that over to the new plugin's API (and in fact, I have that mostly done for macOS and Windows). Did you have an issue with that code?

[jpnurmi]

I see. I was simply not aware of the other implementation.

[jpnurmi]

No reason to spend time reviewing this since an implementation already exists.