Show total size of package download required in Add/Remove Applications.

Unlike synaptic or "update manager", "Add/Remove applications" does not shows the amount(size) package download required. Showing the amount of package download required and the amount of free space used after unpacking can be very useful to the end users.

With out this info, this can cause problems with the users with slow internet speeds or the users with limited amount of internet download quota.
It can even cause the problems when there is no enough free space in the root partition.

By knowing this info, users can avoid above said problems.