Allows to download concurrently multiple docsets.

Adds: stopping of downloads.
Fix: user does not need to change tabs to reset the listview.
Moves menu items to the ui class
Implements back and forward actions and buttons.
Refactors the search query.

The search query now precomputes the field instead of computing them on every function call.
Fix: Does not add a docset if the download failed.

If due to the network error the docset download failed does not add it to the docsets list.