Thread Changes Cleanup

[Aidan63]

This contains various bits of cleanup related to the c++11 threading changes from the last few merges.

• All the old __hxcpp_ functions are now excluded unless building for haxe 4 (it's a shame we can't conditionally remove that old Thread.cpp file from the build xml).
• Various internal uses of HxCreateDetachedThread have been replaced with a detached std::thread so that function can be removed.
• std::this_thread::sleep_for is now used internally instead of #ifdef's for various OS'
• You can use the HXCPP_THREAD_XML define to point to a build xml file which will provide implementations for the hx::thread api instead of the built in one.
• Added a few tests for the Scratch type which was added in the typed thread PR.

I've put some more thoughts on the structure of these threading changes in this issue #1279