Diagnostics: Switch state to new system

Add an additional state to the cmake instance to track diagnostics. Use
this to initialize any new/reset states we create so that other contexts
will inherit the diagnostic state from presets / command-line options.
Modify the front-end to map presets and -W command-line options to
manipulation of this new diagnostic state. Remove old members for
storing diagnostic state. Rework how diagnostic state is persisted.
Deprecate old, non-standard spellings for manipulating diagnostics.
Deprecate old 'dev' spelling for CMD_AUTHOR.

Note that this is a work in progress. Messaging has not yet been
updated, so the new state is not yet being used to control diagnostics,
and some of the logic to update the old state has been stripped (and
will eventually be removed entirely). Backward compatibility support for
the deprecated CMAKE_{WARN,ERROR}_DEPRECATED variables is only partially
implemented. The GUI's interface for manipulating warnings is partially
broken and needs to be overhauled. These tasks are being done in
separate commits in order to minimize review burden and keep the changes
per commit / per merge request more manageable.