Save test project before closing: add 'cancel' option to MessageBox

[rowo360]

Describe the bug
If a test project is closed which were created newly or modified, we display this MessageBox to the user, if he wants to save his changes.

I propose that we add a 'cancel' option to this MessgeBox, so that the user can abort his operation.
Especially when closing TestCentric application, this 'cancel' option might be helpful to revisit the changes

To Reproduce
Steps to reproduce the behavior:

1. Create new test project
2. Close test project or Close TestCentric application
3. MessageBox is shown, if test project should be saved before closing

Environment (please complete the following information):

• TestCentric Version:2.0.0-beta6