[kcm/kwinrules] Fix Placement setting

Summary:
Since the port to KConfigXT, the setting "placement" is stored using the enum value, instead of a string equivalent.
This updates the kcm model to be able to communicate this property with the config schema.

BUG: 421586
FIXED-IN: 5.19.0

Test Plan:
- In a rule set the "Initial Placement" property
- The value is stored in `~/.config/kwinrulesrc`
- Close/reopen the kcm. The placement value is correctly loaded.

Reviewers: #kwin, #plasma, zzag

Reviewed By: #kwin, #plasma, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29790