Like the previous change, this allows Qt to handle proper translations
of the UI buttons, rather than us needing to handle it.

Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.

yuzu/applets/software_keyboard: Override accept() and reject() instead of providing own differently named member functions

Uses Qt's built-in interface instead of rolling our own separate one on
top of it. This also fixes a bug in reject() where we were calling
accept() instead of reject().

std::function instances can potentially allocate. std::moveing them
prevents an avoidable allocation in that case.

a

Allows using Qt provider over default.

Implements all of the features of the keyboard, including length, default text, character validation, and UTF-16 character support.