Previously when compiling, the following deprecation warning would be
encountered:
```
src/mainwindow.cpp:447:70: warning: ‘QList<T> QObject::findChildren(const QRegExp&, Qt::FindChildOptions) const [with T = QLineEdit*; Qt::FindChildOptions = QFlags<Qt::FindChildOption>]’ is deprecated: Use findChildren(const QRegularExpression &, ...) instead. [-Wdeprecated-declarations]
```
This commit updates this line to the non-deprecated QT5 version of this
method, which uses QRegularExpression rather than QRegExp, thus removing
the warning.
