x11-themes/slim-themes: fix invalid option definitions

Defining PORT_OPTIONS directly, especially after makefile fragment
inclusions, is not valid and results in missing options values (yet masked
when they are evaluated later after they are needed).

Rework the automagic to function as intended with the exception of removing
the redundant and potentially misleading "ALL_THEMES" option. All the
themes are already set on by default and to change that required the extra
step of unchecking ALL_THEMES (without doing so still resulted in all
being selected even when options are unchecked which is confusing).

Finally, use the option framework to enforce one choice with MULTI option
rather than roll-your-own logic.

PR: 211569
Approved by: maintainer timeout
Discovered by: Synth sanity check failure