Redisplay after spawning browser

After launching a browser child process, gertty's display may no
longer be current (e.g., when using a console-based browser).
Redisplay the screen contents following browser execution as a
prophylactic measure.

Note: you can test this by invoking `BROWSER=lynx gertty` with lynx
installed (but really, can you call it a computer if it doesn't have
lynx installed?!?).

Change-Id: I929e6ec4252b3e4c7c8c425a87cc8be7c0434828

gertty/app.py

@@ -525,6 +525,7 @@ class App(object):
     def openURL(self, url):
         self.log.debug("Open URL %s" % url)
         webbrowser.open_new_tab(url)
+        self.loop.screen.clear()
 
     def time(self, dt):
         utc = dt.replace(tzinfo=dateutil.tz.tzutc())