Add icon, desktop file for the Linux versions of the setup program on 0.7.x

[stephengtuggy]

Thank you for submitting a pull request and becoming a contributor to Vega Strike: Upon the Coldest Sea.

Please answer the following:

Code Changes:

• [Not yet ] Have the PR Validation Tests been run? See https://github.com/vegastrike/Vega-Strike-Engine-Source/wiki/Pull-Request-Validation
• [ ] This is a documentation change only

Issues:

• Please list any related issues

Purpose:

• What is this pull request trying to do? Port #42 to 0.7.x
• What release is this for? 0.7.x
• Is there a project or milestone we should apply this to? 0.7.x