Make sure these headers can be compiled even when the control they're
written for is not available in the build. Including them in this case
doesn't make much sense, of course, but not giving any errors is still
nicer and consistent with the rest of wx headers.
This is more correct as saving the current width of the last column
would prevent the user from shrinking it under the last automatically
set size, i.e. the UI behaviour would change after restarting the
program, which shouldn't be the case.
Doing this required making WXGetManuallySetWidth(), which previously
existed in the generic version only, available in all ports, so do it
and also rename it to WXGetSpecifiedWidth() in the process, as this
seems a somewhat better name (it doesn't have to be manually specified,
i.e. it could also be done by the program itself or even implicitly by
wxPersistentDataViewCtrl).
Don't make this function public, at least for now, because it's not
clear how could it be useful and it might still need to be changed to
behave differently in the other ports.
