KCM LookAndFeel check widgetStyle availability before applying it

Summary:
BUG: 419552
* Some Global Theme may refer to breeze widget style with a lower case b, apply the proper name `Breeze`

BUG: 418698
* Some Global Theme use widget style that may not be installed, then don't apply it.

Test Plan:
* Open KCM LookAndFeel, open GHNS and install Qogir-light theme, then apply it. Open Application Style KCM, if kvantum is not installed, Breeze is highlighted.
* Open KCM LookAndFeel, open GHNS and install Aritim-Light, then apply it. Open Application Style KCM, Breeze is highlighted.

Reviewers: #plasma, ervin, bport, meven, ngraham

Reviewed By: ervin, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28662