applets/systemtray: fix crash caused by model index mismatch
Reason for the change
Though QSortFilterProxyModel only has one source model, it doesn't mean the source model can use the model index from QSortFilterProxyModel directly. During reordering the source index doesn't necessarily match the proxy index.
Test plan
Screenshots or screen recordings
Bugs fixed
SENTRY: PLASMA-WORKSPACE-13MR