Port Weboob to KF5

Kross doesn't want to run python scripts anymore, so Weboob plugin is
dead. This patch makes it alive again.

Changes:

1. no KF5::Kross dependency, as it is deprecated,
2. new Python2 dependency, as it's all we need to run weboob,
3. if no weboob is installed then plugin won't be compiled neither,
4. renamed weboob.py to kmymoneyweboob.py and put it in kmymoney/weboob

directory to avoid name clashes,

5. renamed class names to better represent what they are for,
6. introduced little exception handling from python scripts and

informational dialogs for that,

7. made single private class out of two in mapaccount, it is mystery why

two of them were needed before,

8. avoided crashes/never ending progress bars if no weboob

backends/accounts were loaded.

Differential Revision: https://phabricator.kde.org/D9855