Add invokable public API for emitting contextualActionsAboutToShow

Summary:
This signal is emitted just before showing the context menu to allow
lazy-loading expensive actions like hasAlternatives.

This emitted by our code whenever we show a context menu from an event
filter, however we want to use this code from a tooltip in D17410

Test Plan:
Hovered over some applets, they correctly enabled/disabled the
alternatives button

Reviewers: Plasma

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20417