Use Kirgami.ToolBarApplicationHeader instead of the view headers

Summary:
Use the standard Kirigami component as header for Discover. Will need
some polishing on the kirigami component but will pay off in the long
run.

CCBUG: 385973
Supports T7958: Adopt Kirigami ToolBarApplicationHeader

Reviewers: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10471