Multiple docset downloads, back/forward buttons.

Add: Allows to quickly go back and forward to previously browsed docsets.

Add: Clicking on the download button now downloads all of the selected docsets in the list. You can select multiple docsets by ctrl clicking multiple items, holding the mouse button down while selecting multiple items or clicking on an item and then clicking on another while holding shift.

Fix: if the download of a docset fails due to a network error it is no longer added to the installed docsets list, hence you no longer need to delete the docset first in order to retry the install.