explorer: add option to cancel file feature extraction

[mike-hunhoff]

Enable users to cancel capa explorer analysis during file feature extraction step. This really only matters for very large files with a lot of file features.

Presently capa explorer displays a Cancel button but clicking it does nothing because we are not tracking individual extraction steps like we do while extracting function/basic block features. We will need to add this capability in order for users to be able to cancel.