MainWindow InfoBar cleanup
Clean up MainWindow's InfoBar implementation:
- Add a common base class for info bars that implements the status/description pattern
- Add a stack widget for managing info bars
- Rework info bars used for account status reports and account problem reports to use new common impl
- Make MainWindow use the stack so only at most one info bar is shown at a time
Fixes #697 (closed), may help with #406 (closed)