Prevent winit from overriding LSUIElement in package manifests

[purajit]

Fixes #261

Essentially, MacOS users of applications that use winit were unable to set LSUIElement in their
package manifest to make the app behave as an agent app, even if the application itself wasn't
enforcing anything. This was because winit was using a default value of ActivationPolicy:Regular
if none was provided, overriding anything in the package manifest.

So, make with_activation_policy override package manifest only if explicitly specified.

See ActivationPolicy docs, LSUIElement docs.

Tested with a branch off of v0.30.4, since v0.30.5 (clean) breaks the build of the application I'm using.

• 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
• Updated feature matrix, if new features were added or implemented

[madsmtm]

Looks good with comments resolved, thanks!

[purajit]

@madsmtm addressed your comments, and thanks for the quick review! I've tested with a branch
off of v0.30.4 since v0.30.5 (clean) breaks the application I'm testing it through. Do you think
this is good enough? If not, I can try doing a test with the example tomorrow.

[madsmtm]

Looks good, thanks! And yeah, your testing is good enough.

[purajit]

Awesome, thanks!