Set Program Arguments (from UI or settings)

Somehow related to [issue #21](https://github.com/spywhere/Javatar/issues/21).

If this pull request goes through, I will update the [projects](https://github.com/spywhere/JavatarDoc/blob/master/projects.rst) section of the documentation as well.

I feel like setting execution parameters is an operation that would happen more frequently than setting dependencies / source folder, so I thought it would be better to put the option before the others.

I did not add the option for global parameters (in the UI), because I could not think of a moment where it would make sense to set default parameters like that globally (or at least, it should happen very rarely, and in that case it can be set manually in the settings and should still work).

If you think the code / messages are not correct, disagree with my choice of `show_input_panel` or would like to see the feature implemented differently,  please let me know.