A warning configuration is a list of command line (warning) options for
libclang. Three non-editable built-in configurations are provided by
default. The user can copy a configuration to customize it.

This is still a global setting and it changes take effect after
re-opening a document. Both issues will be addressed in follow-up
changes.

Change-Id: I86667d7dc39ad31b88666454220e6da563797740
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>