Add confirmation dialog to change VRRP address.

This patch causes a confirmation dialog to rise when change VRRP
address button is launched.

To achieve that, this patch changes the class of the button to button_confirm,
where confirm dialog logic is already implemented.
Add progress bar to all confirm buttons.

All buttons asking for confirmation show a progress bar if confirmation is given.
Progress bar is displayed until the background call finishes.
Merge branch 'develop' of git://github.com/dana-i2cat/opennaas into feature/confirm-and-progress-bar-in-gui-buttons
Merge pull request #546 from isartcanyameres/feature/confirm-and-progress-bar-in-gui-buttons

Feature/confirm and progress bar in gui buttons