Use batch for feature enabling

Problem/Motivation

Currently Feature Set uses module_enable() to enable selected feature sets. Given that multiple feature sets may be enabled at once and that each feature set may include multiple dependencies, it would be safer to trigger a batch operation for enabling the features and their dependencies, as is done in core at install time for modules enabled by an install profile.

Proposed resolution

See the batch operation in subprofiles for a possible model: http://drupalcode.org/project/subprofiles.git/blob/refs/heads/7.x-1.x:/s....

See also #1488030: Merge with Subprofiles?.

Remaining tasks

User interface changes

Users enabling feature sets would see a progress bar.

API changes

None.