Improvements on the contract views.

- [FIX] A simplified tree view in module reconcile_compassion was
        overwriting normal tree views. Now it is a separate view.
- [ADD] Frequency in contract tree view
- [ADD] Contracts in partner view are sorted on start_date
- [ADD] Contracts in partner view can be opened on current window.
- [FIX] PEP8 errors.