Add an API and backends for system notifications

[Kontrabant]

Notifications are asynchronous, passive popups that convey some information to the user. This adds support for various native system notifications and an associated SDL API.

Tested and supported platforms:

• *nix via D-Bus interfaces (both the "core" interface and a portal for use within containers is supported)
• macOS (10.14+, only supported when used within app bundles due to internal OS requirements)
• Windows (Win10+, technically Win8 supported notifications, but some newer interfaces that require Win10 are used, and Win8 usage is basically zero at this point)

Needs testing:

• iOS (builds, but currently untested. My old MacBook can't run a new enough XCode to target my phone)

Possible future backends:

• Android (this is certainly doable, but I have zero experience with Android and no devices to test it on.)
• Emscripten (browsers support notification functionality, so this is probably doable?)

All implementations currently support some basic features in addition to header and body text:

• An attached image
• Actions for user feedback (presentation is system dependent, but usually buttons or menu items)
• Custom sounds (some platforms restrict where the sound files can be located, with Windows being particularly restrictive)

Feedback from actions comes in the form of a new event, which returns the invoked action string specified when the notification was dispatched. It can also return a "default" action, if the notification is interacted with in a non-specific way (e.g. tapping or clicking on the body instead of a button).

New capabilities can be easily added by adding additional properties. A simple interface that just takes a title, body, and image, is included as well.

The Windows implementation also provides a situation where SDL_wcstoul() is needed, so that was added in a separate commit, along with associated tests.

Closes #7857 (supersedes)
Closes #1932