1729-Window level always done one first image

I changed the update() method in medViewPropertiesToolBox to select item corresponding to the current layer instead of selecting item 0. 

It seems a bit strange because there was a comment saying that we didn't have the current layer information, although we do have it...
The lines I modified were introduced in commit e90ff200411d0eb7642af9a8ce37d0d4afed43ef ("Selects the new layer as current layer, or 0 when deleting it.").

Guillaume