Core: Refactor file dropping into reusable component

Change-Id: I42d291d8f8c0e76e289e56055044c369e8be8a89
Reviewed-by: David Schulz <david.schulz@digia.com>
EditorManager: Support dropping files on specific split

It was opening files in whatever split was "active".
As a side effect this also fixes dropping files on extra editor windows
(which did nothing).

Change-Id: I9db9f731eba8718313d3cc43e35609676bcffdc3
Task-number: QTCREATORBUG-9023
Reviewed-by: David Schulz <david.schulz@digia.com>