Rename "Close Edit Mode" tooltip to "Finish Customizing Layout"

Summary: We don't call it the "edit mode" anywhere else in the UI. It now also matches the menu entry.

Test Plan: Hover over the desktop toolbox close button.

Reviewers: Plasma, VDG, ngraham, mart

Reviewed By: Plasma, VDG, ngraham, mart

Subscribers: mart, ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24855