macosx: show the localized, intended to be user-facing encoding names instead of the variable values to the user.

This way, s/he is never presented an empty menu item for the default value.

This also closes #7539 by preventing stringWithUTF8String calls with empty strings.
(cherry picked from commit d7246593be4f2484d13adaaf5bd8cbd4c8e5b812)

Conflicts:
	modules/gui/macosx/open.m