Add before-input-event event for webContents (fixes #7586)

Add before-input-event event for webContents (fixes #7586)

Embedding arbitrary web content is problematic when it comes to keyboard
shortcuts because:

* Web content can steal app shortcuts (see e.g. brave/browser-laptop#4408)

* Blocked web content (e.g. a focused <webview> performing expensive
computation) will also prevent app shortcuts from firing immediately

The new before-input-event event can be used to overcome these issues by
always handle certain keyboard events in the main process.

Note that this requires electron/brightray#261 to compile.