Fix rearranging items in places panel with hidden items

Summary:
`m_sourceModel->movePlace` is called using index from the view model.
This becomes an issue when there are hidden places items.

This change calls `m_sourceModel->movePlace` using the corresponding mapped source index.

BUG: 399430
FIXED-IN: 19.04.0

Test Plan:
  # Create at least 4 Places items
  # Hide first item
  # Drag the now first visible item between second and third item

Actual: The item does not move
Expected: The first visible item should now be second visible item in the list

Reviewers: #dolphin, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: ngraham, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19901