[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