Replace variant ImeRequest::Disable with fn disable_ime

[dhardy]

• [ ] Tested on all platforms changed
• [ ] Added an entry to the changelog module if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [ ] Created or updated an example program if it would help users understand this functionality

This is the first part of #4412. It could be merged as-is or in concert with other changes.

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Summary

Replace ImeRequest::Disable with fn disable_ime.

Motivation

It was documented that ImeRequest::Disable cannot fail, yet it returned a Result. This revision removes the API ambiguity.

[dhardy]

Would you prefer to merge changes piecemeal (just this commit) or shall I push more IME changes into this PR?

The plan is roughly outlined here.

[kchibisov]

I think I'll also look myself into it one more time and probably push some of the changes myself.

[dhardy]

As you like.

I already made further changes (not pushed yet): move winit::window::Ime* types to new winit_core::ime module, drop Ime prefix for these types.

[kchibisov]

If you split them into commits, it's fine with me. I can tune based on that.

[dhardy]

Pushed.

My further plans are less concrete:

• Split enable / update into separate requests. For the interface I think this makes sense but I'm hesistant because these ops share a lot of code in the backends.
• Replace the error for update_ime with NotEnabled. Or possibly make it infallible ignoring errors.
• Add ChangeCause parameter to update_ime
• Implement From<ImeRequestData> for ImeEnableRequest to make usage a little easier, while keeping the existing path
• Make hint and purpose required in enable_ime. However, this means that ImeRequestData cannot be used for both enable_ime and update_ime.
• Revise documentation

[kchibisov]

I think that update and enable should be together, since they are similar and the actual change is semantics, especially given that certain requests only apply on enable in some cases.