OSX: Make StatusBar transparent to clear up visual oddity

Test Plan:
Verify This doesn't look bad on other platforms.

Per https://www.kdevelop.org/tags/kdevelop-5 it seems that Windows and Linux all depend on transparent status bar.

Reviewers: KDevelop, mwolff, kfunk

Subscribers: kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D656