Implement Clone for 'static events

[semtexzv]

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
  Not applicable:
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

There is no reason why 'static events can't be Clone. This unblocks most use cases for cloning safe events while not changing current design significantly.

Blocking WASM + OpenGL implementation for Amethyst ( See amethyst/amethyst#2040)

[est31]

Ideally, we should wait for #1456 and then just add a derive impl.

[semtexzv]

I agree, current design is not ideal, but if no other approach is found, this is a way to get Clone safely for events.

[azriel91]

Heya, is it okay to get this merged (and perhaps published) before a big Event refactor?

Since a refactor would be a breaking change anyway, I don't see any harm in allowing downstream projects to depend on a published version with this change, with the prospect that they have to migrate to the new Event structure (which they have to do anyway).

[Osspial]

Sorry about the delay. This looks good to merge - thanks for pulling it together!