So that temporary system metrics are not saved in the registry. Saving
them to the registry creates a race condition when two processes are
trying to activate theming at the same time, one process might save
themed system metrics instead of unthemed system metrics to the registry.
The race condition will be more apparent when initializing a wine prefix
after theming is turned on by default in wine.inf.

This patch refactors the system metric helper functions to use an
in-memory struct and is a prerequisite to remove the race condition.
Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>