Output the extracted number from wxString::ToXXX() even if it returns false.

Output the extracted number from wxString::ToXXX() even if it returns false.

After the changes in r50710 wxString numeric conversion functions didn't
update their output parameter any more if the conversion failed because not
entire string was converted. This was incompatible with the old behaviour
which some existing code did rely on, so restore it and now always return the
number which was extracted from the beginning of the string if we found
anything at all, even if the function returns false.

Add unit test for the correct behaviour and updated the documentation.

Closes #11126.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61786 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775