Always use value argument for the text control contents and also override the
initial numeric value with it if it's numeric.
This seems to be the only consistent thing to do, so document this behaviour
and add a unit test checking for it.
Closes#13589.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71387 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775