This diff adds support for creating multi-volume rar and 7z archives. A QDoubleSpinBox was added to CompressionOptionsWidget which allows setting the volume size in megabytes between 0.1 to 1000. The size in megabytes is converted by CompressionOptionsWidget to kilobytes because 7z doesn't support volumesizes with decimals.
Creating a multi-volume archive changes the archive name (name.part01.rar and name.7z.001) so we need to re-open the archive (the first volume) after adding files.
We only support adding files once, so add/delete actions are disabled in Part if archive is multi-volume and non-empty.