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

Simplify the code by extracting local variables for commonly used things.

No functional changes!
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

Add containers for <wicket:head> to the HtmlHeader(Items)Container in the page
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

Copy <wicket:head> contents right after </wicket:header-items> when <wicket:header-items> is used.
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear