On KCM cursor theme update attribute name

Summary:
Rename m_model to m_themeModel and selectedIndex to themeSelectedIndex
to avoid confusion because we have two models in this class.

Reviewers: #plasma, mart, ervin, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24518