`CGEventTap` helpers?

[madsmtm]

CGEventTap seems non-trivial to use, so maybe we should add APIs to help with that, similar to what core-foundation do? Though may also be smart to not do so, since it apparently has a few soundness difficulties.

See:

• Add CGEventTap servo/core-foundation-rs#436
• CGEventTap: Allow dropping events from callback servo/core-foundation-rs#492
• Incorrect behavior of CGEventTap servo/core-foundation-rs#664
• Cancel event when CGEventTap callback returns nullptr servo/core-foundation-rs#665
• [breaking][core-graphics] Fix unsoundness in CGEventTap API servo/core-foundation-rs#710
• [breaking][core-graphics] Allow dropping event from CGEventTap callback servo/core-foundation-rs#724
• [breaking][core-graphics] Enforce Sendness of CGEventTap callback (soundness fix) servo/core-foundation-rs#725