QUnicodeTables: mark all functions as nothrow

Yes, this is necessary. The noexcept operator looks for noexcept tagging,
not at the contents of the function to determine whether to return true.

The more conditionally-noexcept functions are used, the more important it
becomes that low-level functions are correctly marked noexcept. In that, it
is like constexpr.

Change-Id: I4bca178444d1fd7caf3a92f996b1536eebdb5014
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>