Improve keyboard-only usage of the toolbar

Summary:
Have PrivateActionToolButton trigger its action if the mnemonic is activated.

BUG: 403711

Test Plan: I can call the actions on the toolbar now (e.g. on Discover application page)

Reviewers: #kirigami, mart

Reviewed By: #kirigami, mart

Subscribers: plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D21188