Web: Use mouse events instead of pointer events if the latter isn't supported

[hayashi-stl]

Pointer events aren't supported on Safari yet, so this pull request uses mouse events when pointer events aren't supported on the web.

• 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
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[ryanisaacg]

Thanks for the PR! Two quick nits:

• Could you address the compilation warnings that this change introduces?
• Would you like to do the same change for the stdweb backend?

[hayashi-stl]

I'm not experienced with stdweb, so I'm not as confident about fixing the problem for that backend.

[alvinhochun]

Pointer events aren't supported on Safari yet

According to https://caniuse.com/#feat=pointer, Safari 13 does support pointer events.