[plasma-dialer] Reorganize QML components, cleanup sidebar-related QML files

Follows the kclock approach by Devin:

• https://invent.kde.org/plasma-mobile/kclock/-/commit/415e46ed67f8c7ac66f578bf5dc8582e7238077e
• https://invent.kde.org/plasma-mobile/kclock/-/commit/b25f1639dcc0be630401c1c56f763efc691abf1e
• https://invent.kde.org/plasma-mobile/kclock/-/commit/d20f96f56e966592a5ee916108e001615fe5cffa