Rename Icinga\Web\Form::onShow() to onRequest()

onShow() is misleading because at the time this method is being called the
form is neither shown nor created.

refs #5525
Fix spelling issue in german translation catalog
Add class ReadablePathValidator
Add the FormErrors decorator as default decorator for forms

This allows us to use Zend_Form::addError() to show error messages for
the entire form context.

refs #5525
Fix that the force_creation checkbox disappears after another error

Once the user enters invalid data after he tried to save a backend without
success (because the backend was not successfully validated) the shown
checkbox disappeared in this case regardless of whether it was checked
or not.

refs #5525
Rename ResourceForm and make it use handleRequest() & Co.

refs #5525
Remove BaseConfigController, User\Message and Widget\AlertMesageBox

refs #6758
ConfigController: Remove obsolete functionality