[compiz] Tweaks to plugin dependency rules

[Mike Dransfield]

It seems to me that the plugin dependencies are not flexible
enough for most plugins.

The particular problem I am thinking about is animation/group
and fade.  They both specify RuleAfter but compiz interprets
that as 'load after AND require it', there is a Require rule, but
this relates to features not plugins.

I would like to propose these changes and definitions, they seem
more logical unless I am missing something.

CompPluginRuleBefore - If this plugin is loaded, make sure I am
loaded before it.

CompPluginRuleAfter - If this plugin is loaded, make sure I am
loaded after

CompPluginRuleRequire - Require this plugin to be loaded if I
am loaded.  This can be combined with other rules.  This is roughly
what RuleAfter is now.

CompPluginRuleRequireFeature - Requires a feature to be
loaded.

What do you think?  RuleBefore might be tricky to implement
because it is checked retrospectively.  It would probably take
2 passes for each check, but maybe we could find a better way.