E-book viewer: Keyboard shortcuts for Back/Forward

Fixes #1186928 [Missing keyboard shortcut in the viewer](https://bugs.launchpad.net/calibre/+bug/1186928)

src/calibre/gui2/viewer/documentview.py

@@ -1218,6 +1218,12 @@ class DocumentView(QWebView): # {{{
                 self.paged_col_scroll()
             else:
                 self.scroll_by(x=15)
+        elif key == 'Back':
+            if self.manager is not None:
+                self.manager.back(None)
+        elif key == 'Forward':
+            if self.manager is not None:
+                self.manager.forward(None)
         else:
             handled = False
         return handled

src/calibre/gui2/viewer/keys.py

@@ -44,4 +44,10 @@ SHORTCUTS = {
         'Right' : (['L', 'Right'],
             _('Scroll right')),
 
+        'Back': (['Alt+Left'],
+            _('Back')),
+
+        'Forward': (['Alt+Right'],
+            _('Forward')),
+
 }