Properly handle ProcessAttribute enabled and default visibility

Summary:
Rather than enabling them always, use column and application visibility
to determnine if an attribute should be enabled. In addition, use a
new visibleByDefault property to set the default visibility state of
columns.

Test Plan:
Start ksysguard, see no plugin provided columns but they can be enabled
from the context menu.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24053