Instead of having separate warning functions (and globals) for each
different warning that can be raised, encapsulate this logic into
a single class and allow to (un)set any number of warnings.
Introduces behaviour change:
- the `-alertnotify` command is executed for all
`KernelNotifications::warningSet` calls, which now also covers the
`LARGE_WORK_INVALID_CHAIN` warning.
- previously, warnings were returned based on a predetermined order,
e.g. with the "pre-release test build" warning always first. This
is no longer the case, and Warnings::GetMessages() will return
messages sorted by the id of the warning.
Removes warnings.cpp from kernel.
