DispatchDialogue : Use Close instead of Ok

[chrisc-lee]

Users sometimes click the Ok button, thinking it will take them back to be able to run the dispatch again. Using Close instead of Ok here removes the ambiguity.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.