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