Fix modifier keys not being sent on Firefox

[theduck32]

This commit allows for modifier keys to be sent on Firefox, even if privacy.resistFingerprinting is enabled or not.

Firefox does set the modifiers in the event itself but when privacy.resistFingerprinting is enabled it doesn't send them, therefore this code handles that issue and sends it manually allowing for key combos like Ctrl+Alt+Delete to work out of the box.

I've tested this on Firefox and Librewolf with both privacy.resistFingerprinting enabled and disabled, both give the same intended result.

This also fixes #1882

[CendioOssman]

Thank you for your contribution. I'm afraid that I'm very sceptical about doing anything here. This is a hidden feature in Firefox, that they readily document as breaking websites. Adding complexity and risk to other noVNC users does not seem like a good trade-off in that case.

I would suggest continuing to put pressure on Firefox to diagnose and resolve this on their end.