Add comments to explain in more details what the function does and in particular the valid ranges of all internal variables; fix in that regard both the code of both wxELLIPSIZE_START, wxELLIPSIZE_MIDDLE and wxELLIPSIZE_END. Add more asserts to check the valid ranges and turn a couple of time-expensive checks in level-2 asserts. Add a test unit for the wxControl::Ellipsize function. Fix minor details in the docs of wxControl::Ellipsize. Closes #11567. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63660 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775