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