Removing prioritizations one-by-one can be tedious. This commit enables
extended selection in the prioritizations list. Multiple items can be
selected with conventional methods, such as holding down Ctrl or Shift
key and clicking the items or holding down the left mouse button and
hovering the cursor over the list. All items also can be selected with
Multiple items drag-n-drop is also possible.
To avoid confusion, the selection in the prioritizations list is cleared
after the items are removed or drag-n-dropped.
Signed-off-by: Sergey Zhuravlevich <[email protected]>