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 !