Get rid of QDeclarativeMetaType::{canCopy,copy}

Get rid of QDeclarativeMetaType::{canCopy,copy}

Now that we have QMetaType::construct() that does placement new
construction, we can use that to copy the value.

We need to destruct the (default-constructed) existing value first,
but for primitive types that's a no-op, and for Qt's types it's
cheap since they use lazy initialization or "shared null".

Change-Id: Idadee04b1d5b590be7fec50fb0396fd277bee973
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>