Apparently we've been getting away so far without this essential #include only
by "leakage" from other #includes in existing consumers. <eyeroll/>

MAINT-6570: Changes for LLCheckedHandle plus some timeout issues from AndreyK regarding script compiles/resets.

I recently tried to wade through llhandle.h and got somewhat perplexed. Armed
with an explanation from Richard, I've added notes to the file to try to make
it a bit less mysterious.

Leaving llhandle.h in llui restricts the set of viewer project directories
which could potentially use it, and there's nothing whatsoever UI-specific
about it.

also improved selection behavior for object contents when deleting/adding items

fixed handle downcast not initiating handle

can be downcast to LLHandle<Derived> using the LLHandleProvider mixin

also made handle subtyping work

moved a bunch of template class statics to cpp files
also added filename parameter to LLXUIParser::readXUI for better debugging of XUI errors

reviewed by James

Merging revisions 127913-128319 of svn+ssh://svn.lindenlab.com/svn/linden/branches/skinning/skinning-17 into D:\viewer-2.0.0-3, respecting ancestry