Add a key to return to the project list

To facilitate escaping from a deep stack of screens, add a key
to clear the history and return to the main project list.  Bind
this to meta-home by default.

Change-Id: Ic98e47d7a3a17271bf21230ec4bac184f691ced3

gertty/app.py

@@ -500,6 +500,9 @@ class App(object):
         commands = self.config.keymap.getCommands(key)
         if keymap.PREV_SCREEN in commands:
             self.backScreen()
+        elif keymap.TOP_SCREEN in commands:
+            self.clearHistory()
+            self.refresh(force=True)
         elif keymap.HELP in commands:
             self.help()
         elif keymap.QUIT in commands:

gertty/keymap.py

@@ -31,6 +31,7 @@ CURSOR_MAX_RIGHT = urwid.CURSOR_MAX_RIGHT
 ACTIVATE = urwid.ACTIVATE
 # Global gertty commands:
 PREV_SCREEN = 'previous screen'
+TOP_SCREEN = 'top screen'
 HELP = 'help'
 QUIT = 'quit'
 CHANGE_SEARCH = 'change search'
@@ -81,6 +82,7 @@ DEFAULT_KEYMAP = {
     ACTIVATE: 'enter',
 
     PREV_SCREEN: 'esc',
+    TOP_SCREEN: 'meta home',
     HELP: ['f1', '?'],
     QUIT: 'ctrl q',
     CHANGE_SEARCH: 'ctrl o',

gertty/mywid.py

@@ -21,6 +21,8 @@ GLOBAL_HELP = (
      "Display help"),
     (keymap.PREV_SCREEN,
      "Back to previous screen"),
+    (keymap.TOP_SCREEN,
+     "Back to project list"),
     (keymap.QUIT,
      "Quit Gertty"),
     (keymap.CHANGE_SEARCH,