Add progress bar element to maxp dialog

Added the element to the GUI and set the signal to update the value
when the float was emitted.
Send signal to the worker to update progressbar

The Worker is now able to update the progressbar's value at any
moment in the execution.

The progress at the moment is just to indicate a moment at the execution
rather than an actual estimate of the real work done.
Merge pull request #5 from sergiobuj/progressbar

Progressbar