Bug#407696: add a guided partitioning option for resizing an existing partition

["peter green" <plugwash@P10Link.net>]

package: debian-installer

currently to use guided partitioning on a system with no unpartitioned free space the user must go into manual partitioning, resize the existing partition and then go back out of manual partitioning and select guided using the largest free space, this is somewhat unintuitive.

since resizing an existing partition to make room for debian is likely to be one of the most common ways of installing it i think it deserves a seperate option at the same level as the guided-use entire disk and guided-use largest free space options.