Extend model with cross entries of transactions

Issue: #22
Update to latest Babel update site (translations)
Add missing dialogs to create portfolio transactions

Issue: #22
Display offset account in lists of transactions

Issue: #22
Check model for missing cross entries

Issue: #22
Dialog for the user to fix reported consistency issues

Issue: #22
Add tests for cross entry objects

Issue: #22
Remove obsolete toString methods
Launch product using the product file

The launch configuration is platform-specific and therefore not shareable.
Check that a reference account exists for a portfolio
Add shares and quote information to security performance view

Issue: #49
Fix charts beginning at 1970 if no CPI information is available

Issue: #52
Add performance number to portfolio performance overview
Show account context menu at all convenient locations
Show security context menu at all convenient locations
Delete cross entries when deleting an account / portfolio
Run consistency check after import
Move context menu classes to avoid package cycles
Missing translation
Bug: deposits on the last day of reporting interval are not performance neutral