Fix style
Clarify documentation
Enhance unit tests
Make it possible to disable operations

- Added activityMode property to PiiOperation
- PiiOperationCompound can now automatically disable failing operations
  and operations that depend on them or other disabled operations.
- Added ErrorHandling flag to PiiEngine::execute()
- PiiNullInputController is now used by disabled operations to prevent
  them from blocking senders.

Minor changes
- Made PiiOperation::State a read-only property with a notify signal
- Changed the signature of stateChanged signal. Affected some demos.
- Added PiiOperation::errorString() and hasError()
Add IgnoreFail flag and fix start of unconnected operations
Fix style
Fix test-related issues

PiiOperationTest incorrectly handled the ExpectFail flag after previous
changes.

PiiOperation.state is now a property. Changed QML tests accordingly.

Fixed QML test for PiiOperation.reconfigure