Easier to customize icons on message widget

[erikn69]

CSS change, works the same, but on custom tabs extending messages widget is easy to set icons
Just add label class like this:

div.phpdebugbar-widgets-messages span.phpdebugbar-widgets-my_custom_label:before {
    content: "\f058";
}

Where my_custom_label is the message label

[erikn69]

adds emergency, alert, critical, notice, debug from Prs\LogLevel standar

php-debugbar/src/DebugBar/Bridge/SlimCollector.php

Lines 50 to 57 in d20048d

	Log::EMERGENCY => LogLevel::EMERGENCY,
	Log::ALERT => LogLevel::ALERT,
	Log::CRITICAL => LogLevel::CRITICAL,
	Log::ERROR => LogLevel::ERROR,
	Log::WARN => LogLevel::WARNING,
	Log::NOTICE => LogLevel::NOTICE,
	Log::INFO => LogLevel::INFO,
	Log::DEBUG => LogLevel::DEBUG