GUI: move Import1D/2D buttons to "File" menu
|Status:||Backlog||Start date:||17 May 2019|
Currently, the Import1D/2D buttons reside in the "Data" view (per #2365).
Their functionality, however, does not depend on the data view being active.
Therefore they could be moved to the top-level "File" menu, in accord with the
wide-spread practice to have all import and export functionality bundled there.