Bump gtk to 0.16

[pentamassiv]

Thank you for writing relm. A new version of gtk and their related crates was released. This PR allows using it. The minimal Rust version is now 1.63.0 (due to gio). There are two caveats though, but they only apply to some of the examples:

• The webkit-test example no longer works and was disabled in the Github Workflow. The reason for this is that webkit2gtk needs to publish a new version which is compatible with gtk 0.16.
• The gladis crate was deprecated so there will not be a version that is compatible with gtk 0.16. I removed the gladis example. The replacement for this crate are CompositeTemplate from gtk3-macros, but I did not write an example with that.

I also updated a few other dependencies in the examples so they should all be up-to-date now :)

BTW, the code could use a cargo fmt. I would have done it, but it would probably be easier for you to run it so you don't need to review the changes.

[antoyo]

BTW, the code could use a cargo fmt. I would have done it, but it would probably be easier for you to run it so you don't need to review the changes.

I don't like the formatting of cargo fmt, so could you please revert the formatting changes? If that would help, you could add a rustfmt config file that prevents doing any formatting.

[pentamassiv]

I reverted the formatting changes and added a rustfmt.toml file so others don't try to auto-format the code. The PR is ready for your review :)