gtkgui: Import urlib.parse instead of urllib

[adrienverge]

Because:

>>> import urllib
>>> urllib.parse.unquote('file://test')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'urllib' has no attribute 'parse'
>>> import urllib.parse
>>> urllib.parse.unquote('file://test')
'file://test'

Closes https://github.com/adrienverge/PhotoCollage/issues/109