Add wxNativeWindow allowing to easily embed native widgets in wx.

Implement the class for wxMSW, wxGTK and wxOSX/Cocoa, show it in the widgets
sample and add documentation for it.

samples/widgets/icons/native.xpm

@@ -0,0 +1,81 @@
+/* XPM */
+static const char *const native_xpm[] = {
+/* columns rows colors chars-per-pixel */
+"16 16 59 1 ",
+"  c #E40000",
+". c #E50000",
+"X c #E70000",
+"o c #EA0000",
+"O c #EB0000",
+"+ c #ED0000",
+"@ c #F10000",
+"# c #F20000",
+"$ c #F60000",
+"% c #FA0000",
+"& c #FB0000",
+"* c #FC0000",
+"= c #FC0001",
+"- c #FE0002",
+"; c #FF2C35",
+": c #FF4F4F",
+"> c #75ED00",
+", c #76ED00",
+"< c #76EE00",
+"1 c #76F000",
+"2 c #77F100",
+"3 c #79F600",
+"4 c #7AF700",
+"5 c #7BF700",
+"6 c #7AF900",
+"7 c #7BF900",
+"8 c #7CFB00",
+"9 c #7DFC00",
+"0 c #7EFE00",
+"q c #81FF04",
+"w c #82FF07",
+"e c #8AFF17",
+"r c #8CFF1B",
+"t c #8CFF1D",
+"y c #A7FF15",
+"u c #92FF26",
+"i c #92FF27",
+"p c #93FF29",
+"a c #9AFF37",
+"s c #9CFF3B",
+"d c #A1FF46",
+"f c #A1FF4D",
+"g c #AAFF56",
+"h c #A4FF5C",
+"j c #A2FF68",
+"k c #B0FF62",
+"l c #B6FF71",
+"z c #B8FF73",
+"x c #C5FF67",
+"c c #FFA6A6",
+"v c #FFB4B4",
+"b c #C0FF83",
+"n c #CBFF98",
+"m c #E2FFB3",
+"M c #B5D9FF",
+"N c #D2E6FF",
+"B c #D2E7FF",
+"V c #DBECFF",
+"C c None",
+/* pixels */
+"CCCCCCVVMNCCCCCC",
+"CCCBBBBBBBBBCCCC",
+"CNNNNNNNNNNNNNCC",
+"C-MMMMMMMMMMMMVC",
+"CXoMMMMMMMMMMjsC",
+"Co  ;MMMMMh9>>qC",
+"C+X X%Mfq7>>>>iC",
+"C#    y>57>>>>dC",
+"C$   Xy6pe>>>>gC",
+"C% X$%x3>3i2>>kC",
+"C:%%X y>>>>t7>lC",
+"Cco   y>>>>>9tbC",
+"CCv+  y>>>>>2gCC",
+"CCCC# y>>>9zCCCC",
+"CCCCC%y>anCCCCCC",
+"CCCCCCmCCCCCCCCC"
+};