Fixes several bugs in the clipboard code, and makes some structural
improvements:
- Adds scoped wrappers for clipboard open/close and global lock/unlock,
to prevent missing cleanup, fixing at least one case where the lock
was not released.
- Adds the relevant window handle to the clipboard calls, since the docs
suggest that some operations won't work without one.
- Adds a missing clear step to setting the clipboard data.
- Switches from TEXT to UNICODETEXT to handle non-ASCII text correctly.
- To enable that, adds UTF-16/-8 conversion utilities built on the
Win32 APIs (rather than the deprecated std::codecvt functions, as
have been previously used in the engine).
- Fixes handling of getting data when the clipboard is empty, correctly
returning null.
- Passes more errors back through the method channel, with details, for
easier debugging of future issues.
Fixes https://github.com/flutter/flutter/issues/54226
