Overhaul network activity toggle

- Rename RPC command "togglenetwork" to "setnetworkactive (true|false)"
- Add simple test case
- GUI toggle added to connections icon in statusbar

src/Makefile.qt.include

@@ -264,6 +264,7 @@ RES_ICONS = \
   qt/res/icons/key.png \
   qt/res/icons/lock_closed.png \
   qt/res/icons/lock_open.png \
+  qt/res/icons/network_disabled.png \
   qt/res/icons/open.png \
   qt/res/icons/overview.png \
   qt/res/icons/quit.png \