DolphinTabWidget: cleanup index-by-URL API usage

Summary:
Follow-up of D23655 where we didn't have time to polish the code:

- add a couple of variables to make the code more clear.
- remove the `get` prefix which we usually don't use in Qt code.
- make the function private since it's very tied to implementation.
- add a new isUrlOpen public method as wrapper.

Reviewers: feverfew

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D23860