Fixes #1654: Fixed the issue that a new message source object is generated for every new message being translated

The signature for `yii\gridview\ActionColumn::urlCreator` is changed - the `$action` parameter is moved to the first

Fixes #1610: `Html::activeCheckboxList()` and `Html::activeRadioList()` will submit an empty string if no checkbox/radio is selected

PgAdmin generates   sequence as  '"Schema"."Table_seq"'::regclass  when
non public schema is used.

Fixes #1499: Added `ActionColumn::controller` property to support customizing the controller for handling GridView actions

I checked this using XDebug, and the function actually returns null (which is something different than false). I assume it is the comment that should be changed, and not the code itself...

Renamed `attributeName` and `className` to `targetAttribute` and `targetClass` for `UniqueValidator` and `ExistValidator`.
Refactored UniqueValidator and ExistValidator.