windows: Add testnet icon for testnet link

Overhauled testnet icon by Jonas Schnelli

src/Makefile.qt.include

@@ -230,6 +230,7 @@ RES_ICONS = \
   qt/res/icons/about.png \
   qt/res/icons/about_qt.png \
   qt/res/icons/bitcoin.ico \
+  qt/res/icons/bitcoin_testnet.ico \
   qt/res/icons/bitcoin.png \
   qt/res/icons/chevron.png \
   qt/res/icons/clock1.png \

src/qt/res/bitcoin-qt-res.rc

@@ -1,4 +1,5 @@
 IDI_ICON1 ICON DISCARDABLE "icons/bitcoin.ico"
+IDI_ICON2 ICON DISCARDABLE "icons/bitcoin_testnet.ico"
 
 #include <windows.h>             // needed for VERSIONINFO
 #include "../../clientversion.h" // holds the needed client version information