Include extended partitions in the count of active partitions

Trying to create a new partition table on a device with active
partitions reports the number of active partitions in the error dialog.
However when there is a busy logical partition the number of reported
busy partitions will be one less than the number of partitions in the
main UI showing the busy symbol.

GParted considers extended partitions as busy when any of the logical
partitions it contains as busy.  Display in the main UI reflects this.

Fix Win_GParted::active_partitions_on_device_count() to not exclude
extended partitions from the count.