Modified the update UI to become more intuitive.

There is now a one-click method of upgrading all packages. When the updater is first opened, the available updates aren't all selected.

This allows the user to quickly open-close the updater to see if anything interesting is ready to be updated, without being prompted to 'discard changes'.

Clicking the apply-all button applies all available updates. The user is still able to cherry pickupdates then click 'apply' or 'ok' as previously possible.