settle issue of KeyboardModifier bit availability

There are currently an unclear number of bits potentially available for expanding KeyboardModifier with additional Super/Hyper/etc flags:

1. 0x8000'0000, which might not be possible (need to stay in positive i32?) but is already in KeyboardModifierMask (0xfe00'0000).
2. 0x0020'0000, 0x0040'0000, and 0x0080'0000, which are noted as available in commit 7931d96 for explicitly this purpose, but with caveats around supporting Qt::Key_unknown with additional modifier flags.

Can decisions around these be made and documented in the source?

I hope that moving (or special case not supporting newer modifiers with) Key_unknown would be tractable. Not locating Qt::Key's custom range in 0x001(1...f)'xxxx right above RFC 3629's limit already unnecessarily cost one potential modifier bit, and it would be a bit disappointing if Key_unknown's enum value cost another 3 by itself.