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.

(cherry picked from commit 920779a980d9873eb077652a2e20a45d168d15d9)
WICKET-5771 Ability to escape resource bundle messages added with wicket:message

Remove wicket-1.5.xsd.
Update version of wicket.xsd to 7.0.0
Remove obsolete DTDs