Add release notes for 6.4.0
Merge remote-tracking branch 'origin/master'
Start next development version
Move bean validation to experimental section

Makes it easier to upgrade all experimental modules after a release
Manage experimental module versions from parent

Three Poms for the core project under the sky,
Seven for the Integrations in their XML of DOM,
Nine for Experimentals doomed to die,
One for the Maven parent POM,
In the Land of Maven where the Snapshots lie.
One POM to rule them all, One POM to find them,
One POM to bring them all and in the classpath bind them
In the Land of Maven where the Snapshots lie.
Fix the remaining POM
minor javadoc corrections, replaced dot with colon for component path
separator in javadoc and tests
WICKET-4935 removeCommonPrefix now removes empty leading segments again
WICKET-4933 don't move disabled options
WICKET-4937 Add IResponseFilter that can filter out invalid XML characters
Add licence headers
WICKET-4937 Add IResponseFilter that can filter out invalid XML characters

Add logging
Use Args to check the arguments
WICKET-4937 Add IResponseFilter that can filter out invalid XML characters

Export the root element name in XmlAjaxResponse as constrants and use these constants in XmlCleaningResponseFilter
Use dependencyManagement from the parent.
test session existence first to prevent unnecessary creation of
temporary session
WICKET-4937 Add IResponseFilter that can filter out invalid XML characters

Do not append invalid xml characters
WICKET-4948 fix vertical centering of modals in safari
WICKET-4608 small optimization for String vars
=WICKET-4932: testing recreateMountedPagesAfterExpiry before to recreate a mounted page