Allow disabling unsafe implicit conversions in wxString

While we have to keep these conversions enabled by default, they are very
dangerous as they can result in silent data loss on any system not using a
locale with UTF-8 encoding, i.e. always under MSW.

Allow mitigating this by defining wxNO_UNSAFE_WXSTRING_CONV when compiling the
application code using the library, which makes these conversions invisible to
the user code, and so can be used without recompiling the library.

Also add wxUSE_UNSAFE_WXSTRING_CONV which can be set to 0 when compiling the
library to disable these conversions globally for all applications using it.

Closes #11830.

docs/doxygen/mainpages/const_cpp.h

@@ -286,6 +286,14 @@ more details.
          wxMSW-specific setting which can be set to 1 to make
          wxWindow::GetCharWidth() and wxWindow::GetCharHeight() more compatible
          with old wxWidgets versions. Changing it is not recommended.}
+@itemdef{wxUSE_UNSAFE_WXSTRING_CONV,
+         this option determines if unsafe implicit conversions of wxString to
+         @c char* or @c std::string (depending on whether @c wxUSE_STL is 0 or
+         1) are defined. It is set to 1 by default for compatibility reasons,
+         however it is recommended to set it to 0 for the new projects. See
+         also @c wxNO_UNSAFE_WXSTRING_CONV below for an alternative way of
+         disabling these unsafe conversions not requiring rebuilding the
+         library.}
 @endDefList
 
 @section page_cppconst_miscellaneous Miscellaneous
@@ -326,6 +334,15 @@ more details.
         don't include compiler flags needed for multithreaded code generation. This
         implies that wxUSE_THREADS is 0 and also that other (non-wx-based) threading
         packages cannot be used neither.}
+@itemdef{wxNO_UNSAFE_WXSTRING_CONV,
+        this symbol is not defined by wxWidgets itself, but can be defined by
+        the applications using the library to disable unsafe implicit
+        conversions in wxString class. This is especially useful when using
+        standard build of the library, e.g. installed by the system package
+        manager under Unix, which is compiled with @c wxUSE_UNSAFE_WXSTRING_CONV
+        set to 1 for compatibility reasons as @c -DwxNO_UNSAFE_WXSTRING_CONV
+        can be used only compiling the application code, without rebuilding the
+        library. Support for this option appeared in wxWidgets 3.1.1.}
 @itemdef{WXMAKINGDLL_XXX,
         used internally and defined when building the
         library @c XXX as a DLL; when a monolithic wxWidgets build is used only a