Add support for custom display components to Action

Summary:
This adds a "displayComponent" property to Action, which allows specifying a
component to use when displaying an Action. Support is added to ActionToolBar
and ActionsMenu for displaying these custom components.

Demo:

Test Plan:
ActionToolBar and ActionsMenu still work the same. The new Action Tool Bar page
in Kirigami Gallery displays the custom components. (See D25941)

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: mart, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25940