Fix function wxControlBase::DoEllipsizeSingleLine to really make sure that the ellips...

Fix function wxControlBase::DoEllipsizeSingleLine to really make sure that the ellipsized string takes less pixels than maxFinalWidthPx.
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