add try_ctx_for_window function

[jakobhellermann]

This is necessary for bevy-inspector-egui to support multiple windows. Could you maybe release a small non-breaking version so that I can implement this without waiting for the next big release?

[mvlabat]

What if we just change ctx_for_window to return an option? I'm personally fine with making a new minor release with this breaking change. I think it's easier to do so rather than making a patch release just to remove the try flavour later.

[jakobhellermann]

In most cases ctx_for_window would be unwrapped if it returned an option. When creating a new window and doing .add_system_set(SystemSet::on_update(AppState::Done).with_system(ui_second_window.system())) the UI system will only run when the window exists.

My use case is for adding a ui system to a Plugin which runs even if the window is not set up yet.

[jakobhellermann]

I think having both methods is fine, because the panicking version will be used the most, and the try_ version exists as a fallback for the more remaining cases. Same with vec[idx] vs vec.get(idx).

[mvlabat]

Sounds fair. And thanks for the PR, I'll release the new version today.