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

[Kontrabant]

Fixed a few memory leaks, and revamped the data structure used to add "actions" to notifications, so it can be easily extended in the future to support additional types.

The basic design here is complete, so I'm removing draft status.

[slouken]

I haven't done a thorough review, but in general this looks good to me. @icculus, thoughts?