Add cancel button for each of the ongoing docset downloads

[nishanthkarthik]

Preview

The Cancel button cancels the current download by emitting a cancelButtonClicked signal. This signal is emitted by overriding QStyledItemDelegate::editorEvent and checking if the QMouseEvent coordinates fall inside the cancel button bounds

[nishanthkarthik]

This fixes #712

[trollixx]

Thanks for the contribution!

After a quick test I've noticed that the Cancel button stays visible during installation stage, but does not do anything.

There are two options here: a) hide the button, b) make cancellation work for installation. The second requires more work, and should be submitted as a separate PR first.

[trollixx]

Additional comments.

Will rebase/merge on top of master.

[trollixx]

It's been a while, and sorry for ignoring this for so long. I believe this should go in, and can be polished later. The base branch has changed a lot, and some files do not even exist any more. Rebasing is pretty much impossible, but copied changes seem to work as expected. Let me know if you want to update the PR yourself, otherwise I'll push my version (with attribution) in a couple of days.

[trollixx]

I've force-pushed the original changes without the insanity of merge commits.