QRegExp: fix \i \I \c \C \p \P escape sequences

QRegExp: fix \i \I \c \C \p \P escape sequences

Those escape sequences have a special meaning in the XML Schema 1.1
regular expressions, but not in Perl-compatible ones.

An escape sequence that has no special meaning should match the
escaped character itself; this patch fixes QRegExp's behaviour in
that regard (previously, it added a character class matching
nothing).

Change-Id: I983f923baa7c2ec19938b96353f3a205e6c06d58
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>