Add .desktop file generation for Linux built apps/games

[DanS2D]

There should be a checkbox added to the Linux build window, that allows the user to get an automatically generated .desktop file for their game/app.

It should pick up the icon file from the projects root directory to use in the desktop file.

For those who do not know, .desktop files are the Linux equivalent to desktop shortcuts in Windows/Mac. They allow you to launch a program, pin the launcher to your taskbar, have the program show up in search results etc.