[kcms/keyboard] Restore metadata for the KDED module

Before the conversion, kcm_keyboard.desktop was used by the KCM and keyboard.desktop by the KDED module. However, the conversion deleted both and only created keyboard.json for the KCM, also referenced by the KDED module.

Effectively, the KDED module ended up with KCM metadata instead and wasn't loaded as it should. With this, the module has the correct metadata again and works as expected again.

This commit brings back keyboard.desktop from before the removal, converts it to JSON and renames it to kded_keyboard.json to avoid future mixups.

CC @butirsky