Post-Annotator: add option to delete multiple annotations

There already has been an issue for that (#831), but since it was a user request that came up in the last weeks a bunch of times, I decided to create a new "to do" issue with a more concrete suggestion how to implement this.

I think compared to the Files page, the option to select multiple annotations should not be offered by default. I suggest to add a menu entry for the annotation items in addition to the "delete" option: "delete multiple". Only if you select this option the checkboxes in the annotation items show up and you can select those you want to delete and confirm with "delete selected". There should be the option to "cancel" this operation. If you "select that"cancel", the checkboxes should be hidden again.