QUnicodeTables generator: Minor clean-up & stricter checks

Use static_cast instead of C-style cast;
ushort -> unsigned short;
Avoid a lvalue in qGetProp();
Minor styling fixes.

Change-Id: Ic6412e28ac9316a587d32d0e674d21f672967fd6
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>