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(gcmh): New group (previously referenced but not defined).
Remove now redundant references to this group.

Feature/prevent duplicate timers

See merge request !2

I found that when combined with desktop-save-mode, upon session
reloading in a new Emacs instance, then gcmh-mode would be enabled
multiple times even when it was already active. This causes multiple
duplicate idle timers to be created (one for each time the mode is
enabled).

The same effect can be provoked by manually running (gcmh-mode t)
multiple times.

The effect of this is garbage collection being executed multiple times
each time the idle time expired.

I noticed this because GC was slower than expected, and turning on
gcmh-verbose revealed all those duplicate GC events each time Emacs
had been idle.

This change simply removes any extant timer before enabling the mode
so we're guaranteed to only have one or zero idle timers defined.

Update conventions

See merge request !1

First removed the unnecessary require statements. Then update some of
the wording to be clearer. Reworked the minor mode to be cleaner and
more consise. Split out the idle timer function to its own definition.
Also removed the setting of garbage-collection-messages. It may not be
desired behavior and does not disable with the minor mode. If the user
wants to set it, they can do so.