Change SimpleLayout to be immutable

[snakefoot]

Remove the Text-property-setter, and the ConfigurationItemFactory-instance-field.

This will allow one to create SimpleLayout without involving the ConfigurationItemFactory.

Introduce fluent-api for creating SimpleLayout from chain of layoutrenderers (skip parsing and reflection-logic)

[snakefoot]

Notice there is a hurdle with handling dynamic assignment of text-property:

    <layout xsi:type="LayoutWithHeaderAndFooter">
        <header xsi:type="SimpleLayout" text="&lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot; ?&gt;&lt;root xmlns:log4j=&quot;http://jakarta.apache.org/log4j/&quot;&gt;"/>
        <layout xsi:type="Log4JXmlEventLayout" />
        <footer xsi:type="SimpleLayout" text="&lt;/root&gt;"/>
    </layout>