Task progress calculation

The subtasks progress now is calculated with hard-coded "weight" of each of subtasks that takes part in the entire operation.
This is very inflexible - for each combination of subtasks, some other set of those values are needed (imagine md5 checksumming that might be enabled by a single entry in configuration - it would require different weights).
It seems a better idea to just present user what really was done (this will be ok for progress percentage, less for count of files/size of files; although we may not need those information for entire task - maybe those informations for subtask will be enough).