[kitemviews] Change the search in Desktop Behavior/Activities to more in line with other search labels

Summary: This patch removes the "Search:" portion of Activities (in System Settings, Desktop Behavior --> Workspace) and adds "Search..." to the search field.

Reviewers: VDG, Plasma, kde-frameworks-devel, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D18046