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!

(cherry picked from commit 685f04cfcbf9983fb0a143bd2500b09e2c79ee4c)
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

(cherry picked from commit c31f30f6914b5d9bd146864015d5aebe16cd681c)
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.

(cherry picked from commit 686030c6763baf24fb8a8f3b24a396369d6f16e1)
WICKET-5531 Create new placeholder tag to indicate where header contributions should appear

(cherry picked from commit c0d2ef035efbcf9244206507731764372a121372)