Implemented merged injected-settings

"site.ini/ExtensionSettings/ActiveExtensions[]" = array( 'extension' )

Won't apply when getting a group, and is overridden by a non-merge
key (without []). Should support associative arrays as well.
EZP-22210: Syntax
EZP-22210: Refactored injected merge settings
EZP-22210: phpdoc

[skip ci]
Merge pull request #880 from ezsystems/EZP-22210-legacy_bundles

Implemented injected-merge-settings