Optimize Formula::scan and make it close to double as fast.

Regular expression using QRegExp are indeed evil. This improves the time spend in Formula::scan during loading the www.worldmapper.org%2Fdata%2Funderlying%2FWorldmapper_U3_hdr%304_tables1-22.ods from ~4.6 seconds to ~2.8 seconds what saves us around ~2 seconds.
Tested with cstester and some Q_ASSERT's against all our ods, xls and xlsx documents to make sure the just introduced Util::isCellReference behaves exactly like the regular expression did before.