WICKET-5416 BOM in UTF markup file breaks encoding detection

Copy BOMInputStream, ByteOrderMark and ProxyInputStream from commons-io 2.4 (http://svn.apache.org/viewvc/commons/proper/io/trunk/src/main/java/org/apache/commons/io/input/)
WICKET-5418 properly handle groups in NonNull constraint
WICKET-5418 properly handle groups in NonNull constraint
Revert due to bad formatting "WICKET-5418 properly handle groups in NonNull constraint"

This reverts commit 7bd677ff843fdfece3963e303faaf901aa5ec04b.
Revert due to bad formatting "WICKET-5418 properly handle groups in NonNull constraint"

This reverts commit 49922cdd5ccba3b97c59106d3aea769b8468962e.
WICKET-5418 properly handle groups in NonNull constraint
WICKET-5420 Make UploadProgressBar easier to extend

Always remove the custom attribute Wicket sets to form element when calling its onsubmit() method.
onsubmit() may throw an exception and the attribute won't be removed.
WICKET-5420 Make UploadProgressBar easier to extend

Allow to set the value of the 'display' property with Wicket.DOM.show()
WICKET-5420 Make UploadProgressBar easier to extend

Fix a typo in comments
WICKET-5420 Make UploadProgressBar easier to extend
WICKET-5420 Make UploadProgressBar easier to extend

Change the type of the bar and status components to MarkupContainer so they can have children if needed
wtpSupport