[Eclipse] Prevent duplicate template source URLs from being added

[keithchong]

Codewind version: latest
OS: Mac

IDE extension version: latest
IDE version: Any supported Eclipse version

Description:
The Add template source wizard allows you to add 'duplicate' URL locations but different names and descriptions. When you finish with the manage template source dialog to call cwctl to update templates, you will get an error:

Error: Bad Request - URL is already a template repository

Suggestion: prevent the user from adding the duplicate URL from the wizard.