Bug#824603: meld gives python traceback on first use after installation.

[Mark Sumner]

Hi,
I had this issue using version 3.16.0-1 and it still occurs for me now that I
have updated to 3.16.2-1:

2:~$ mkdir /tmp/fake-home meld
2:~$ cp .Xauthority /tmp/fake-home/
2:~$ HOME=/tmp/fake-home meld
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/meld/meldapp.py", line 72, in do_startup
    self.new_window()
  File "/usr/lib/python2.7/dist-packages/meld/meldapp.py", line 138, in new_window
    window = meldwindow.MeldWindow()
  File "/usr/lib/python2.7/dist-packages/meld/meldwindow.py", line 266, in __init__
    self.widget.set_help_overlay(shortcut_window)
AttributeError: 'ApplicationWindow' object has no attribute 'set_help_overlay'
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/meld/meldapp.py", line 78, in do_command_line
    tab = self.parse_args(command_line)
  File "/usr/lib/python2.7/dist-packages/meld/meldapp.py", line 336, in parse_args
    focus=i == 0)
  File "/usr/lib/python2.7/dist-packages/meld/meldapp.py", line 149, in open_files
    window = self.get_meld_window()
  File "/usr/lib/python2.7/dist-packages/meld/meldapp.py", line 144, in get_meld_window
    return self.get_active_window().meldwindow
AttributeError: 'NoneType' object has no attribute 'meldwindow'