Officially Deprecate EventBodyQt.

[TTWNO]

While it may remain part of the public API, invoking its use should elicit a warning about incompatibility with future versions of at-spi2-core and mention the Qt no longer uses this signature. We may also want to change the name to LegacyEventBodyQt, or something similar to indicate its deprecation.

Qt is now officially using the correct signature like GTK does[1], so I'm wanting to make sure no new uses of this structural variant spread throughout our or others' codebases.

1. https://code.qt.io/cgit/qt/qtbase.git/commit/?id=fdf87fdce1c8b926b1c1ee9254aa4b0fc1840d82

[TTWNO]

Where it is required for our own use, we can use explicit ignoring of deprecation using #[allow(...)].