Block the UI for some mass-change operations

[tnajdek]

We currently have a concept of “Ongoing”, which is used for long-running operations that can happen in the background (file upload, metadata recognition, copying items to another library).

However, we currently allow batched operations that offer no feedback (e.g. moving > 50 items). For these operations, we should probably block the UI while the operation is happening, ideally with a progress bar present. Attempting to close a tab while this happens should trigger a prompt as in #478.