fix(kcm): improve i18n of device combobox

Summary:
The original label cause i18n problems, the parameter %1 is never given. The layout doesn't fit the form. This patch uses Kirigami label and form layout. And fix the i18n message.

Before:

After:

Reviewers: kde-i18n-doc, Plasma, Localization, broulik, romangg

Reviewed By: Plasma, romangg

Subscribers: romangg, broulik, ltoscano, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25459