Allow multi-delete and table delete on evidence table

[JoelAtDeluxe]

This PR provides two new abilities.
First, it enables multi-select on the evidence table, either via a ctrl+click (or shift+click) or by dragging the mouse over multiple rows. When multiple items are selected AND the delete context menu is selected, all of the selected pieces of evidence will be removed. A warning prompt appears before deletion, and an error prompt appears after deletion, if some of the files could not be deleted.

Second, this PR adds the ability to remove all of the currently visible rows in the table, which effectively allows for deleting by whole groups of evidence based on a common condition -- e.g. belonging to a certain operation. Again, a prompt appears before, and conditionally after, in much the same was as a standard item delete.

Lastly, this will also remove directories if the last piece of evidence is removed from that directory.

Note that the error condition for deletes has changed. Previously, a message would popup indicating exactly which file was not deleted. Now, we instead attempt to write a file with a list of all non-deleted files in that batch, and we will instead point the user to this file (if it is written)

Addresses Issues: #48, #49

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