Added support for arbitrary operators to Query::filterWhere, added support for arbitrary operators to Sphinx extension

- Removed `FileLoader` and used `\Twig_Loader_Filesystem` instead.
- Added support of Yii's aliases.
- Added `set()` that allows setting object properties.

Improved checks where substr_compare is used, replaced more substr + comapre cases with substr_compare

Better fix for #3443 that does not break BC and allows widget to be extensible.

`yii\web\Request::cookieValidationKey` is now automatically generated by the installation script for the basic and advanced application templates

Fixes #3443: Fixed `yii\bootstrap\Nav` and `yii\bootstrap\Dropdown` were generating wrong ids for submenus

reverts 45e4973a

fixed outdated example.

Fixes #4051: Renamed `yii\caching\GroupDependency` to `TagDependency` and added support for associating multiple tags to a single cached data item