Creates a new BasicSettings class in common/settings, and forces setting
a default and label for each setting that uses it in common/settings.
Moves defaults and labels from both frontends into common settings.
Creates a helper function in each frontend to facillitate reading the
settings now with the new default and label properties.
Settings::Setting is also now a subclass of Settings::BasicSetting. Also
adds documentation for both Setting and BasicSetting.
QRegularExpression was introduced in Qt 5 as a better replacement for
QRegExp. In Qt 6.0 QRegExp is removed entirely.
To remain forward compatible with Qt 6.0, we can transition over to
using QRegularExpression.
Since this is inside a string literal, backslashes that are part of
regex syntax have to be escaped. But that's ugly, so convert to a raw
string instead.
