Add ability to setup flash messages on action success about admin

[klimov-paul]

Resolved by commit 6e1ff6a

[klimov-paul]

Advanced 9f912c5

[loveorigami]

Hi, Pavel
I am watch your yii2tech/admin repo.
And I see also your last commit.

I am also think about "How better display Flash message?".
I am write module
https://github.com/loveorigami/yii2-notification-wrapper
for display messages from stand-alone and ajax (pjax, etc...) actions.

Hope, It will be usefull for this admin.

Regards

[klimov-paul]

Flash messages representation are covered by Alert widget:
https://github.com/yii2tech/admin/blob/master/widgets/Alert.php

Still you are free to use any other solution for this, yii2tech/admin does not restict this.