Fix for WICKET-4664

Added tests for RequestUtils.decodeParameters() : in order to be able to test it easily without the mocking or starting a complex requestCycle, extracted the static call to getCurrentCharset() as a method parameter. The extracted method is package visible only and visible for tests only.

When RequestUtils.decodeParameters() is called with a queryString beginning with a "?" it is automatically stripped and thus is not reflected into pageParameters.
WICKET-5103 use the same sessionId for #storePage() as used in
SessionEntry#getPage()
WICKET-5166 Relax the requirements for using FilteringHeaderResponse
Disable Clirr until 7.0.0 is released
Remove deprecated methods in JavaScriptFilteredIntoFooterHeaderResponse. Use FilteringHeaderResponse#DEFAULT_HEADER_FILTER_NAME as the new default for header filter.

These changes should not affect user applications so there is no need of migration guide entry
WICKET-5168 Depend on Java 1.7
WICKET-5167 Depend on Servlet API 3.0
WICKET-5167 Depend on Servlet API 3.0
WICKET-5152 Improve Wicket behavior in Servlet 3 async context
WICKET-5167 Depend on Servlet API 3.0

Move the code from WicketStuff-Servlet3
WICKET-5168 Depend on Java 1.7

Set Maven version to 7.0.0-SNAPSHOT
WICKET-4817 Remove SerializableChecker and use CheckingObjectOutputStream + ObjectSerializableChecker
WICKET-4966 Remove outdated BoxBorder
WICKET-4969 RequestCycle#getExceptionRetryCount should be in IRequestCycleSettings
WICKET-4964 Rename Component#renderHead(HtmlHeaderContainer)

Rename Component#renderHead(HtmlHeaderContainer) to #internalRenderHead(HtmlHeaderContainer)
Complement for WICKET-4664

Update on Url#getQueryString(), the result is now on par with the ones returned by HttpServletRequest :

  * does not contain the leading "?"
  * returns null when no query parameter is present
Complement for WICKET-4664

Url#toString() was not reflecting the new behavior for getQueryString()
WICKET-5103 use the same sessionId for #storePage() as used in
SessionEntry#getPage()
WICKET-5166 Relax the requirements for using FilteringHeaderResponse
Disable Clirr until 7.0.0 is released
Remove deprecated methods in JavaScriptFilteredIntoFooterHeaderResponse. Use FilteringHeaderResponse#DEFAULT_HEADER_FILTER_NAME as the new default for header filter.

These changes should not affect user applications so there is no need of migration guide entry