WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

(cherry picked from commit 0123767f798e7715ae0da81188c0e6e2e5f4a236)
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

(cherry picked from commit 8ac36a6c9c1d958b1a7846792e0e7604e0fcd962)
Fix copy/paste error

(cherry picked from commit b32ba89897186526e6f277aca10e379f1f606e79)
Improve the javadoc of HtmlHeaderItemsContainer

(cherry picked from commit 5da01132fdbc4bf8344f5d963230c5f5dd976bbb)
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

(cherry picked from commit a3aec20d2e4c489b26dbd796d9a8efbfe3354b05)
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

Remove a method that is no more needed.
The addition of the closing tag for <wicket:header-items/> is now done in OpenCloseTagExpander

(cherry picked from commit b77ce25776947005377cb6c9496954dc1b9be7f2)
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

Remove the usage of the tag namespace when checking where the tag should be transformed from openClose to open+close.
The namespace is dynamic (it depends) on the xmlns: for the page. We cannot hardcode namespaces in the list of tags to expand

(cherry picked from commit 66fd9d41b24ac4010542d75f4566bf9978f7728b)
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

extend HtmlHeaderSectionHandler javadoc to mention <wicket:header-items/>

(cherry picked from commit 3e053192a1ed9e5def6090e98e92216e997e7925)
Use tabs, as in the rest of the file

(cherry picked from commit ddb7ff3498167b632869e48703042a043d8fdef2)