New Features: Preseed Files and Boot Configurations

Written for Cubic by Cubic PPA on 2019-03-13

Release 2019.03-47 includes two new features for Ubuntu 16.04, 18.04, and 18.10: 1) Edit Preseed files, and 2) Edit Boot Configurations. Gzip compression is used instead of xz compression for lower CPU utilization. This release also includes minor UI tweaks, and Cubic on Ubuntu 16.04 now uses headerbars.