Add support for wxNO_IMPLICIT_WXSTRING_CONV_TO_PTR to wxString

This symbol is similar to the existing wxNO_IMPLICIT_WXSTRING_ENCODING
and can be defined when building the application (as opposed to when
building the library) to disable implicit wxString conversions to
pointer types, i.e. char*, wchat_t* and void*.

This makes the just added wxUSE_CHAR_CONV_IN_WXSTRING library build
option unnecessary, so remove it.

docs/doxygen/mainpages/const_wxusedef.h

@@ -43,7 +43,7 @@ library:
     available.}
 @itemdef{wxUSE_UNSAFE_WXSTRING_CONV, Defined as 1 by default for compatibility,
     but can be set to 0 to globally disable unsafe implicit conversions in
-    wxString class. See @ref overview_string_settings for more details.}
+    wxString class. See @ref string_conv for more details.}
 @endDefList