Implement simple version checker for python plugin's dependencies.
PEP396 declare that module should have a `__version__' attribute
with 3-tuple of integers. Also, some bad modules have it as a string,
which is also handled.

X-Python-Dependencies now may have a version in a parenthesis possible
w/ leading version compare operator ('=' is omitted). The others are:
<, >, !=, <=, >=. Desktop files are fixed for the new style.