general: Make most settings a BasicSetting

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.

src/core/reporter.cpp

@@ -397,7 +397,7 @@ void Reporter::ClearFSAccessLog() const {
 }
 
 bool Reporter::IsReportingEnabled() const {
-    return Settings::values.reporting_services;
+    return Settings::values.reporting_services.GetValue();
 }
 
 } // namespace Core