Can't use Tab/Enter in IME candidates list

[MrJul]

Describe the bug
After #12560, Tab can't be used anymore to select composition candidates, and Enter never commits the composition.

To Reproduce
Switch to Japanese Kana keyboard layout, type a few chars until the IME candidates list appear.
Try to use Tab to select a candidate: Tab is handled by Avalonia instead and the focus moves.
Try to use Enter to validate the IME: Enter is handled by Avalonia instead and inputs a line break.
(And probably other special shortcuts I'm not aware of.)

Expected behavior
While an IME session is active, the keys should go to it.

Remarks
I'm not sure what's the best resolution is here, the reversed TextInput/KeyDown events order before #12560 wasn't ideal but it ensured that special keys were handled by the IME.

There's no IME start/end notification on macOS?

@danwalmsley @Gillibald

[Gillibald]

Maybe we should just process the key-down event all the time? Will investigate.

[kekekeks]

If IME is in play user code should never get events that are processed by IME. Attempting to handle those is invalid behavior which needs to be removed.