[pkg] vendor fix for the separator in windows

vendor/README

 2018-11-14
 -----------------
 * vendor getlantern/systray with PR #74, to work around menu positioning bug in windows.
+* manually add a patch for the 18x18 icon size for mac
+* vendor PR #76 too (add separator to visibleitems)

vendor/github.com/getlantern/systray/systray_windows.go

@@ -508,9 +508,12 @@ func (t *winTray) addSeparatorMenuItem(menuId int32) error {
 
 	mi.Size = uint32(unsafe.Sizeof(mi))
 
+    t.addToVisibleItems(menuId)
+    position := t.getVisibleItemIndex(menuId)
+
 	res, _, err := pInsertMenuItem.Call(
 		uintptr(t.menu),
-		uintptr(menuId),
+		uintptr(position),
 		1,
 		uintptr(unsafe.Pointer(&mi)),
 	)