Maintain selection when changing tabs
Fixes #44 by ensuring that we maintain the docset selection during tab changes.
Merge pull request #75 from ryanwersal/iss44

Maintain selection when changing tabs
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.
20131220 release
Hint about multiple selection in the download dialog
Fixed 'QT' typo
Merged with upstream