Datasets widget: add shortcut for deleting files

The scenario is the following:
1) Open WD.
2) Add "Read sequence" to the scene.
3) Click it.
Expected: datasets widget appears.
4) Add several files.
5) Select some of the added items in the list.
6) Press Delete button.
Expected: the selected files has been removed from the list.
Actual: the element is removed from the scene.