[gdbplugin] Port QRegExp to QRegular Expression

QRegularExpression worked differently, and properly porting to the "new way" would introduce too much changes to the matching. The only way I can come up with was to use variable assignment on the if conditions to (kind of) match the previous paradigm.

I'm not sure if using a static QRegularExpressionMatch (debugview.cpp:282) and assignning to it is the right way to do it, but it is intended to minimize the instantiation of QRegularExpressionMatch.

QRegularExpression::anchoredPattern is used (as per https://doc.qt.io/qt-5/qregularexpression.html#porting-from-qregexp-exactmatch) as I think it better signifies that the whole line has to match, but the same can be done by manually adding the \A and \z to the regex (which solution would be better?).