Proper dependency management in features

Patch attached tries to introduce dependency checking in the Features form.

Might need some work.

It'll enable any additional features it detects as dependencies, if they aren't already enabled. It won't allow a feature to be disabled if another feature depends on it (i.e you cannot disable Migrate until you also disable Clone, if Clone is active).

We should move at least the Disable stuff into hook_validate so that we can set only a form_set_error() without the drupal_set_message() 'The configuration options have been saved' from system module sneaking in.

And if we move it into hook_validate(), we need a way to reset the checkbox states (if you disable Migrate while Clone is enabled, it doesn't disable Migrate and throws the error, but the re-rendering of the form keeps the form state so that Migrate is unchecked: reloading the page altogether fixes it). I think we need form_set_value() here but I'm not a forms wizard.

Also, my earlier patch, though it may have been less elegant, only enables modules that aren't already enabled (i.e it won't enable hosting and hosting_site to enable clone and migrate, if these are already running). I do this with module_exists() in that patch, have to work out a way to replicate this in your work.