Implemented a toggle for showing/hiding the log section of a task

The show/hide status is currently stored inside of the model. This is technically not the right place for the information, but if the value is to be kept within the view then the collection needs to be rewritten to not destroy & recreate all views when a single element is added/removed.
Completion ratio is now based on progress of all actions involved

Each action has a weight. The weight indicates how much of the 100% should be attributed to that action.
Individual actions either contribute their full weight when they have completed, or they can provide more frequent progress updates through the SendorActionContext.completion_ratio callback.
Continuous progress reports for SFTP and parallel-SFTP transfers

Both SFTP and parallel-SFTP transfers will now upate their completion ratio at every 16kB
Bugfixed parallel-SFTP file transfer; it will now transfer large files (>1MB correctly)

The implementation used to use a filemode of 'a'. This made most writes happen at the end of the file (despite the seek operations). Changing filemode to 'r+' seems to help, now seeks/writes happen at the expected locations in the file.