Updated to version 1.4.12 of Page Builder.

* Improved how missing widgets are handled.
* General code clean up.
* Prebuilt layouts are no longer all filtered by siteorigin_panels_data. Filtered by siteorigin_panels_prebuilt_layout when fetched.
* Added more hooks and filters.
* Incompatible plugins now includes more link to give details about incompatibility.

git-svn-id: https://plugins.svn.wordpress.org/siteorigin-panels/trunk@921191 b8457f37-d9ea-0310-8a92-e5e31aec5664