Icon-O-Matic: Remove "using namespace" from header

Putting a "using namespace" statement (_USING_ICON_NAMESPACE in this
case) in a header file causes all source files that include that
header to use that namespace, regardless of whether they actually
wanted to.

Change-Id: I3ebf4ea3366f5f2b9068e3da88f40a7b4d98fd6b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6897
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

Icon-O-Matic: Add reference images

Fixes #2748

Also fixes a comment misstating a function's name and code style.

Change-Id: I609a1f1e100ded647818e70b428cedc48cf29036
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6604
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>