Show action.main more prominently on the ToolBarApplicationHeader

Summary:
Show it as a full button instead of a tool button.
Use the icon color as the button color.

Reviewers: Kirigami, mart, ngraham

Reviewed By: Kirigami, mart

Subscribers: davidedmundson, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D13237