[Linux] Script to create/update .desktop file

[Athanasius]

Ref: https://www.reddit.com/r/EliteDangerous/comments/mtyw2i/prerelease_elite_dangerous_market_connector/gv419wt/?context=3

A .desktop file informs window managers how to launch an application.

Because the EDMC source could be anywhere we can't just supply a static file, we'd need to supply a script that then creates the file.