Update desktop file

[Roboron]

Update desktop file with useful defaults. So it is ready to use by maintainers without further modification (but renaming).

[prissi]

Updated. But what does this file do? It does not go into distribution, does it?

[Roboron]

Desktop files are used by Desktop Environments to expose installed programs to the end user. I guess they can be compared to "Start Menu" entries on Windows. So it is only to be used when installing Simutrans system-wide.

[prissi]

But I think no script includes this file right now. If so, it needs to be inside the simutrans, or not?

[Roboron]

No.