feat: Support ImGui docking

[CrushedPixel]

This addresses #1.

Overview

These are three commits:

1. Added a feature docking that enables imgui/docking feature.

2. Added ImguiContext::with_io_mut to allow users to mutate Imgui's I/O struct. Without being able to modify the I/O struct, there is no way for a user to enable or configure docking.

3. Add a field config_flags to ImguiPlugin that defaults to enabling docking when the docking feature is enabled. This makes it possible to configure Imgui via config flags without having to use with_io_mut.

The third commit is technically a breaking change, as it changes ImguiPlugin's struct layout, so existing code will need to be updated. Therefore, I understand if you don't want to merge this third commit - the first 2 are sufficient to properly use Imgui with docking. Just let me know if I should remove it from this branch.

Demo

With the docking feature enabled, I added a Startup system that runs:

imgui.with_io_mut(|io| {
    io.config_flags |= ConfigFlags::DOCKING_ENABLE;
    io.config_docking_always_tab_bar = true;
});

And now my windows are dockable, without any additional Imgui API calls:

Please let me know your thoughts!

[jbrd]

This looks great, thanks for contributing this :) I'll focus on getting the invalid slice fix through first, and then jump back to reviewing this.

[jbrd]

I haven't forgotten about this one - I'll move onto this next, and I will probably spend a bit more time with it and its likely that it will be part of a more major 0.8.0 release, where breaking changes are more acceptable

[jbrd]

Thanks for your contribution @CrushedPixel, overall I think this looks great. I've left a few pieces of minor feedback. I'd be happy to merge this once addressed...

[jbrd]

This fails to build when the docking feature is not enabled, since ConfigFlags::DOCKING_ENABLE doesn't exist

[jbrd]

Prefer expect over unwrap, so that we can provide a decent error message here (e.g. "Failed to acquire write access to ImGui context"...)

[jbrd]

This unsafe block is unnecessary...

[CrushedPixel]

Thanks, I'll look to address these shortly :)

[jbrd]

Superseded by #68

[jbrd]

Fix published in crate 0.8.0