Feature: Undo test case deletion

[hoanglam-nguyen]

Description of changes made

This PR implements the ability for a user to undo the deletion of a test case if they change their mind. The general procedure is as follows:

• When the user decides to delete a test case, instead of irrevocably removing the test from the UI and cache, the test is marked as deleted in the UI and the cache data is moved to a separate cache for "deleted" test cases
• In place of the bottom panel of the "deleted" test, the user is presented with a button to undo this operation; the original test code is still visible for inspection
• If the user decides to undo the deletion, the test together with its UI is enabled again, and the cache data is moved back to the original cache
• The top bar with the current stats as well as the coverage indicators on the gutter are updated accordingly

Screenshot

Other notes

Closes #230

What is missing?

When the number of deleted test cases is large, they may clutter the list of test cases. In addition to an undo button, one could add a button to permanently remove the test case from the UI.

• I have checked that I am merging into correct branch