Speed up features diff for selected components.

The Features UI nowadays has links to show diffs of only specific components. However, the code internally gets / processes / exportifies / slices / dices all components, before outputting only the partial diff.

It's possible to change some functions (add an extra input parameter for the components to process), to overcome this. This patch will drop processing time to roughly 50% - 70% of what it is now, depending on selected feature and component.

Side effect: admin/structure/features/FEATURE/diff/NONEXISTENT_ID now displays the full diff of the feature; after the patch, it displays the 'regular' access-denied message.

Note that the features.drush.inc part of the patch will fail unless you apply #1429014-3: Filter features diff in drush by component first. But you don't need the drush.inc part if you don't have #1429014 anyway. I will reroll if someone wants to include this patch into Features before #1429014, but I don't expect that.

Code details:

• features_detect_overrides() and features_get_storage() got an extra optional parameter for 'array of components'
• features_access_override_actions() got an extra optional scalar parameter - since it's a menu callback.
• features_access_override_actions() and features_detect_overrides() have internal caching that was per module, and is now broken up into module + component (just like features_get_component_states()). I don't see any performance issues resulting from that.
• So far so good - but I also needed to be able to pass an array of components into features_get_component_states(). I misused the 2nd function argument for that, which is kinda 'meh'. I'd like an opinion before solving the resulting @todo one way or another.

Despite the last thing (one function needs cleanup) I think this should work OK and not backfire on anything.