(Desktop) Set UA via request handler instead of context: Fixes CookieManager

[threethan]

This PR removes the modified request context from desktop webviews.

The modified request context was previously used only to apply WebSettings.customUserAgentString, but had the unintended side-effect of delegating all cookies to the modified CefRequestContext's cookie manager.

• Since a new request context was being created every time a WebView was created, cookies were effectively transient
• Since WebViewState.getCookieManager() uses the global cookie manager, it was unable to access cookies within modified request context

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This PR instead applies the user agent by adding a CefRequestHandler, which does not separate cookies.

This approach still applies the user agent without any apparent downsides.

• The global cookie manager is used, and WebViewState.getCookieManager() works as expected.
• Cookies are now persistent and shared between WebViews on desktop. This seems to be consistent with other platforms.

Resolves #395, #152