Fix crash when creating or editing incidence

Fixed: #262 (closed)

When assigning editor.incidenceWrapper, QML immediately re-evaluates all bindings in the editor. If the wrapper is not yet fully configured, the bindings operate on incorrect data.

I moved wrapper creation and configuration before the assignment to editor.incidenceWrapper. The assignment itself is deferred via Qt.callLater() to the next event loop cycle, so bindings don't fire prematurely.

Also fixed these warnings:

qrc:/qt/qml/org/kde/merkuro/calendar/qml/Dialogs/IncidenceEditorPage.qml:394:17: QML ComboBox: Binding loop detected for property "currentIndex"