OpenMW-CS: Verify global search regexp

Currently we allow to use an arbitrary text as the pattern regEx search:

The problem is that the entered text may be not a valid regular expression. In this case search will not work an it can produce such messages in log:

QRegularExpressionPrivate::doMatch(): called on an invalid QRegularExpression object

Ideally we should verify regular expression when we enter the text (via mRegExp.isValid() method). If the regular expression is not valid, we should block the Search/ReplaceAll buttons and indicate to user that the given regular expression is not valid.