Make winapi crate optional

[rhysd]

This PR makes winapi crate dependency optional.

Context:

• My application doesn't handle image data. So I use this crate without image-data features.
• winapi crate is very inactive (no commit for 3 years). Avoiding it as much as possible would be a good idea.

Without image-data, winapi is used only for syncapi::Sleep in this crate and it can be easily replaced with Rust standard std::thread::sleep. std::thread::sleep uses the same Sleep C function under the hood. As a side effect, std::thread::sleep leverages high-resolution timer. It could make the retry interval more accurate.

This doesn't remove winapi dependency immediately since clipboard-win v4 still depends on it. However clipboard-win will replace winapi dependency with windows-rs at next major version v5. When this crate bumps clipboard-win to v5, winapi dependency can be removed along with this PR (without image-data feature).

[complexspaces]

Thank you for the PR, this seems reasonable to me.

Regarding winapi being used for image functionality, once a new version of clipboard-win is out I can replace winapi with windows-sys here, to remove it from the dependency tree fully.