BaseHtml::listBox() incorrect array_key_exists() usage fix

Html::activeDropDownList() and 'multiple' option fix

BaseHtml::listBox() default size settings removed

Fixes #2003: Added `filter` property to `ExistValidator` and `UniqueValidator` to support adding additional filtering conditions

Fixes #1998 : get input value from hidden field when necessary

Using VarDumper::dumpAsString() instead var_export() in the Request pane...

Some Basic App readme updates [skip ci]

afterFind event in AR is now called after relations have been populated

changed truncate to delete

fixes #1993

Add dutch translations

po filles generation.

Mark event as handled when filter decides to abort action

fixes #1984

Update upgrade-from-v1.md

Text amended as "A new method" was referring to two. CHanged the text to "New methods".