Implement plugin descriptor (read-only)
Implement descriptor save operation
Promote into top level the Settings/Descriptor class

Move Settings into GitlabWebHookRootActionDescriptor. Replace uses of the
class with calls to get the plugin descriptor.
BUGFIX : protect against empty configFile
BUGFIX : wrong code to write to file
Implement true booleans for global configuration
BUGFIX : ruby file closing does not give a return code
Improve XML handling
Improve descriptor standarization