Snapcraft 2.26 no longer builds working snap

[tttppp]

For some reason using snapcraft 2.26 does not build a working snap (or this might be a coincidental change - it might be a latest version of something else). When trying to load an image I get the following exception:

Exception in Tkinter callback
Traceback (most recent call last):
  File "/snap/mountain-tapir/4/usr/lib/python2.7/lib-tk/Tkinter.py", line 1540, in __call__
    return self.func(*args)
  File "mountain_tapir/controller.py", line 106, in <lambda>
    canvas.bind('<Button-1>',lambda e, c=canvas, r=region: self.clicked(c, r))
  File "mountain_tapir/controller.py", line 147, in clicked
    self.recentImages.addImage(imageFile, self.selectPlaceTool)
  File "mountain_tapir/recent_images.py", line 57, in addImage
    imageFile.makeImage('thumbnail', (THUMBNAIL_WIDTH, THUMBNAIL_HEIGHT), imageCellCanvas)
  File "mountain_tapir/image_file.py", line 48, in makeImage
    photoImage = ImageTk.PhotoImage(image)
  File "/snap/mountain-tapir/4/lib/python2.7/site-packages/PIL/ImageTk.py", line 115, in __init__
    self.paste(image)
  File "/snap/mountain-tapir/4/lib/python2.7/site-packages/PIL/ImageTk.py", line 180, in paste
    from PIL import _imagingtk
ImportError: cannot import name _imagingtk

This suggests that something is wrong with PIL. Comparing the prime directory from a working snap (v1.0.1) with a fresh build of the same code I get lots of differences. In particular:

• Changes to the handling of the desktop file including a change to the exec line and removal of the desktop line from the snap.yaml file.
• Missing python scripts from $SNAP/bin (including files named gifmaker.py and painter.py).
• Lots of changes to $SNAP/lib/python2.7/site-packages/PIL
• Pillow 4.0.0 replaced by Pillow 2.9.0.
• Missing olefile and olefile-0.44.dist-info directories.
• setuptools.34.1.1 replaced by setuptools-34.2.0.
• Changes to the mesa copyright (and maybe other files too?)

[tttppp]

The change to snap.yaml is fine (it's actually an improvement): https://bugs.launchpad.net/snapcraft/+bug/1656487

[tttppp]

Also the change in the desktop file from mountain-tapir.mountain-tapir to mountain-tapir is an improvement: https://bugs.launchpad.net/snapcraft/+bug/1658123

[tttppp]

I think there were a couple of issues (see the commit for details):

1. The version of PIL being brought in had changed (I think previously a bug was causing mountain-tapir to work).
2. The app command needed to point to the wrapper.

The new version of snapcraft seems to have also fixed the integration with the application launcher.