Backend should be always totally closed for indexing by search engine robots

we should not provide one in the package but developers should commit
them when starting a new project so that all share the same versions
and only explicit composer update command will change that.
Escpecially important when deploying to production.

This reverts commit a28e7470.

Cleaned up a few comments ... hope its not my OCD again... :-)

Reverted name to ```closeButton``` to be in sync with the bootstrap ```Alert``` widget.

1. Renamed closeButton to closeButtonOptions. 
2. Generate unique id for each alert box.
3. Moved parent init to the top.

Each flash variable can be mapped to a bootstrap alert type through a configuration array passed to this widget.

Cleaned up redundant code (and a typo missed earlier).

Removed redundant declarations and checks in the code.

Ensure previous CSS classes are not appended and reinitialized in the foreach loop.

Used ```Html::addCssClass``` and ```Html::removeCssClass``` more effectively.

Added closeButton parameter, assignment of CSS class corrected, and added php doc comments.

The previous alert widget within the advanced/basic app does not render multiple flash messages. The current implementation allows display of all flash messages assigned through setFlash. Also a validation is done for allowed bootstrap alert types.

Small edit for consistency with /apps/advanced/backend/config/main.php. Sorry, my OCD kicked in.