Added progress-bar functionality to all the tabs.

There is one known bug and a work-around for it:
After running the commands from the 2nd (identify-filenames) or the third one (reports), clicking on the "close" or "cancel" button will freeze the gui. However, the first tab (fiwalk) has no such issues! The work-around till it is fixed is to run "fiwalk" in the first tab before closing. Not neat, but if this is not desired to do, the gui can be exited by pressing cntrl-C.