Fix bugs that could lead to an unresponsive IDE when working with project files

[keyboardDrummer]

Description

• Add option ProjectManagerDatabase.ProjectFileCacheExpiry that allows test to not wait for the cache to expire, something which is otherwise easily forgotten
• Fix a bug that could lead to a non-responsive IDE when a project file was opened and closed
• Fix a bug that could lead to a non-responsive IDE when project file changes were made and undone
• Add defensive coding to prevent disposed project manager from being used (something that occurred in the above two bugs)

How has this been tested?

• Added an IDE test for each fixed bug

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.