kcms/keys (Shortcuts): Refactor and fixes for Qt6/KF6

Use the headers (incl ListView header) for actions.  This gives
the kcm the modern look.  Clean up a few warnings; remove import
versions.

"Add" commands placed in a menu under "New Shortcut" action.

| Before | After |
| ------ | ------ |
|![keys_before](/uploads/f3765742ba6efc12e8d620db225c5477/keys_before.png) | ![keys_main](/uploads/dbc1be9f4916aeed34fe4ebd5a95bdad/keys_main.png)