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