This fixes some crashes in browser panels on Mac, but it's also harmless
if browser panels aren't enabled.

Description

Some parts of Chromium apparently can't live without knowing whether they're being called from the Mac event loop. To satisfy them, this PR implements the isHandlingSendEvent method on NSApp.

Motivation and Context

This fixes a crash led to a crash when you right click a browser panel.

How Has This Been Tested?

Tested in combination with obsproject/obs-browser#197, makes it crash less.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.