#25 Cut and Delete dialogs don't update byte count

[Kimmo Varis]

Frhed 1.3.4.

When user selects area from file and selects either Cut or Delete from context menu the dialog shows:
- begin offset
- end offset (included to cut/delete)
- number of bytes

When the begin- or end-offset are modified, number of bytes is not updated.

This can lead to wrong edits since if user doesn't notice the byte count is not updated and trust he is editing correct size block.