Ubuntu support for start on boot

[jgresham]

Feature details

A .desktop file needs to be created in /home/<user>/.config/autostart/ and kept update with the NiceNode AppImage file path and version.

Example at /home/johns/.config/autostart/nice-node.desktop

[Desktop Entry]
Type=Application
Encoding=UTF-8
Name=NiceNode
Exec=/home/johns/Applications/NiceNode-0.0.6-alpha_bcf6e644054f51b51dd377e42273d2c6.AppImage
Icon=/usr/lib/expressvpn/icon.png
Terminal=false

^ This file path is where AppImageLauncher puts AppImages after "Integrating". The path could be different for each user, so the path needs to found by the main process and kept updated. Ex. The path could also be /home/johns/Downloads/NiceNode-0.5.1-alpha.AppImage. It doesn't appear like the icon path matters (tbd on that).