Support cancelling an in progress BEF project import

[simontoens]

During project import, if the user clicks the cancel button, we should actually abort the project import and clean up.

As an initial implementation, we can check the state of the cancel button between running the various Flows we have around here: https://github.com/salesforce/bazel-eclipse/blob/7850e5b79ac2dc2f941fb4be796429fef87f26c5/plugin-core/src/main/java/com/salesforce/bazel/eclipse/projectimport/flow/FlowProjectImporter.java#L82

Then, a "cancelled/cleanup" method needs to be called on each Flow instance that ran. This method can undo the work a Flow did when it ran.