Modified signal to kill tasks
Implemented clear_previous_results function in case the user chooses
more nodes from the same site. This way, we delete the previous results
and leave the fields blank.
Implemented several functions to manage the buttons behaviour. Improved
behaviour of some of the widgets. Reorganized code.
Fixed bug in gobutton.text. Missing t character.
Added condition in checked_host function, in case the 144 nodes are
selected, the checkall checkbox has to be selected as well
Improved get_selected function to avoid go button enabled with no hosts
selected. Fixes #2
Deleted cancel_task in case of task failure.