Improve ToolTip in the ActionTextField component

Summary:

• Add standard duration (Kirigami.Units)
• Don't display when text is not empty (same behaviour as in Discover)

Test Plan: Manually tooltip appear when not text is present and doesn't appere when text is present

Reviewers: Kirigami, ngraham

Reviewed By: Kirigami, ngraham

Subscribers: ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D19300