Add wxHAS_IMAGE_RESOURCES defined under MSW and Mac

Unlike the existing wxHAS_IMAGES_IN_RESOURCES constant defined only
under MSW, the new one is also defined under Mac and could be defined
for the other platforms/ports later (e.g. wxQt could probably support it
too).

It's unfortunate that two very similar constants are needed, but it
doesn't seem wise to change the meaning of the existing constant, as
this would change how the commonly used wxICON() and wxBITMAP() macros
behave and would break all our own samples that use them for their frame
icon but don't embed this icon into the bundle resources under Mac.

Do change the toolbar sample to use the new constant however, as this
one does include the bitmaps it uses in its bundle under Mac.

docs/doxygen/mainpages/const_cpp.h

@@ -194,8 +194,13 @@ Currently the following symbols exist:
 @itemdef{wxHAS_NATIVE_WINDOW, Defined if wxNativeWindow class is available.}
 @itemdef{wxHAS_NOEXCEPT, Defined if the currently used compiler supports C++11 @c noexcept. @c wxNOEXCEPT is defined as this keyword in this case, and as nothing otherwise.}
 @itemdef{wxHAS_NULLPTR_T, Defined if the currently used compiler supports C++11 @c nullptr.}
+@itemdef{wxHAS_IMAGE_RESOURCES, Defined if images can be embedded into the
+    program as resources, i.e. without being defined in the program text
+    itself. This is currently the case for MSW and Mac platforms. This constant
+    is available since wxWidgets 3.1.6.}
 @itemdef{wxHAS_IMAGES_IN_RESOURCES, Defined if <a href="http://en.wikipedia.org/wiki/Resource_(Windows)">
-    Windows resource files</a> or OS/2 resource files are available on the current platform.}
+    Windows resource files</a> resource files are available on the current platform.
+    Usually wxHAS_IMAGE_RESOURCES should be used instead.}
 @itemdef{wxHAS_POWER_EVENTS, Defined if wxPowerEvent are ever generated on the current platform.}
 @itemdef{wxHAS_RADIO_MENU_ITEMS,
         Defined if the current port supports radio menu items (see wxMenu::AppendRadioItem).}