Windows Icon Defaults to GTK

[blobotic]

On Windows 10, building the examples with MSYS2 (following the steps under "Windows (MinGW + Gtk)" in the "Setup and Installation" documentation), apps are initialized with the default GTK icon rather than no icon, which I presume to be unintended behavior.

Screenshots:
In the title bar (top left corner):

In the Windows taskbar:

[redtide]

Maybe it's just GTK working that way on Windows? Could you try to build and test #266 and see if it could solve the issue?

[redtide]

OK, I think this is not an issue at all because if you run the gtk3-demo.exe appears with the same GTK icon, so I suppose it's a generic replacement icon instead of the classic terminal/console icon (though this is a bit confusing to me because on Linux the GTK2 demo has the GTK icon, the GTK3 demo not, unlike on Windows).

I edited the above pull request to use also a dedicated app icon when the user sets the ELEMENTS_APP_ICON CMake variable, otherwise the Cycfi logo as default instead. Please let us know if this solution is acceptable, or if you have some suggestion if not.