[kstyle] Use QX11Info::isCompositingManagerRunning()

Summary:
Every time Breeze needs to check whether given widget has an alpha
channel, it makes a synchronous X call to figure out whether a
compositing window manager is running on a particular screen. This
is inefficient!

Luckily for us, Qt XCB QPA monitors compositing manager selections
and caches the ownership status of each one. That cached ownership
data can be accessed via QX11Info::isCompositingManagerRunning().

Reviewers: #plasma, hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26978