Feature: #7098 - Severity-filtering for FlashMessageQueue

See Issue #7098

Description

So far only all messages of the FlashMessageQueue could be fetched and/or removed. With this addition it's possible to do that for a given severity only. The existing functions get an optional $severity-parameter.

Usage:

FlashMessageQueue::getAllMessages($severity);
FlashMessageQueue::getAllMessagesAndFlush($severity);
FlashMessageQueue::removeAllFlashMessagesFromSession($severity);
FlashMessageQueue::clear($severity);