Remove almost empty src/generic/icon.cpp

Don't bother creating a separate object file for just a few lines of
code in the generic wxIcon implementation, just inline them into
wx/generic/icon.h.

There should be no need to ever have any non-trivial code here as
generic wxIcon is the same thing as wxBitmap anyhow.

include/wx/generic/icon.h

@@ -19,9 +19,9 @@
 class WXDLLIMPEXP_CORE wxIcon: public wxBitmap
 {
 public:
-    wxIcon();
+    wxIcon() = default;
 
-    wxIcon(const char* const* bits);
+    wxIcon(const char* const* bits) : wxBitmap(bits) { }
 
     // For compatibility with wxMSW where desired size is sometimes required to
     // distinguish between multiple icons in a resource.
@@ -49,7 +49,11 @@ public:
     // create from bitmap (which should have a mask unless it's monochrome):
     // there shouldn't be any implicit bitmap -> icon conversion (i.e. no
     // ctors, assignment operators...), but it's ok to have such function
-    void CopyFromBitmap(const wxBitmap& bmp);
+    void CopyFromBitmap(const wxBitmap& bmp)
+    {
+        if ( &bmp != this )
+            Ref(bmp);
+    }
 
     wxDECLARE_VARIANT_OBJECT_EXPORTED(wxIcon, WXDLLIMPEXP_CORE);