WICKET-5771 Ability to escape resource bundle messages added with wicket:message

Copy wicket-1.5.xsd into wicket.xsd to make it usable even in versions newer than 1.5.x.
Set <schema> version attribute to 6.18.0. IMO we should update this with every change made in this file.
Add 'escape' attribute to <wicket:message> element.
WICKET-5771 Ability to escape resource bundle messages added with wicket:message

<wicket:message> 'escape' will be available in 6.19.0 !
Revert "WICKET-5772 set attached after load, so failuer of load() will not keep null value"

This reverts commit fd3de0085e1f9f0dc98dbf456106d38a4b779f9e.
WICKET-5772 added javadoc
WICKET-5778 Pass the IModifiable to the IChangeListener in ModificationWatcher

Introduce temporary interface IChangeListener2 that can be used when one needs the changed object as a parameter

(cherry picked from commit 2637c1243d061f5d99f0cf10cd3b547359ced187)
init asciidoc
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versions

Conflicts:
	pom.xml
images
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