Add a special symbol which is defined only if the icons and other images (e.g.
cursor) are in the separate resource files and don't need to be embedded as
XPMs in the main program.
This makes the checks more clear and more customizable as it's enough to
change wxHAS_IMAGES_IN_RESOURCES definition instead of changing many platform
checks.
Closes#14050.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70789 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
