Add Signet launch shortcut for Windows

cd src/qt/res/icons
convert bitcoin.png -modulate 100,87,119.4 -define icon:auto-resize="256,48,32,16" bitcoin_signet.ico

This commit also removes the 64-bit mention from testnet.

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

@@ -1,5 +1,6 @@
 IDI_ICON1 ICON DISCARDABLE "icons/bitcoin.ico"
 IDI_ICON2 ICON DISCARDABLE "icons/bitcoin_testnet.ico"
+IDI_ICON3 ICON DISCARDABLE "icons/bitcoin_signet.ico"
 
 #include <windows.h>             // needed for VERSIONINFO
 #include "../../clientversion.h" // holds the needed client version information