Support using closures as DCA callbacks (see #5772)
Add a usage example for assigning closures to templates
Use `NULL` instead of `0` in `tl_files.pid`
Use UUIDs in the theme importer, too
Show error messages if a user is logged into the install tool (see #5001)
Update slimbox to version 1.8 (see #5747)
Use `margin` instead of `padding` for image containers (see #5607)
Add a generic `Model::findMultipleByIds()` method (see #5805)
Pass additional parameters to the "insertTagFlags" hooks (see #5806)
Purge the search index if a page is deleted (see #5897)