Properly handle Unicode clipboard data on Windows.

[nutzboi]

Bug

Currently, copying and pasting non-ASCII characters in STK on Windows is completely broken.
This is because

1. STK ignores the SDL device when on Windows and OSX, and
2. Unicode data in Windows clipboards are encoded in UTF-16 LE while STK attempts to read it as UTF-8.

Fix

This PR fixes it by prioritizing the usage of the SDL device for handling clipboards when available, and converting text data between UTF-8 and UTF-16 LE on Windows otherwise. This also eliminates the need for the overloading clipboard functions with wchar_t types.

Agreement

By creating a pull request in stk-code, you hereby agree to dual-license your contribution as
GNU General Public License version 3 or any later version and
Mozilla Public License version 2 or any later version.

This includes your previous contribution(s) under the same name of contributor.

Keep the above statement in the pull request comment for agreement.

[nutzboi]

Final push; not touching this anymore (edit: oops, typo).
Also, #3751 should have already been fixed with SDL2 like Benau said. Can a linux Wayland user confirm?