gui: fix file / input input command parsing

[ScherrerMichael]

program to run like ls -a returns:

thread 'main' panicked at 'failed to execute process: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/gui.rs:302:10

Currently the input programs and options are manually parsed and separated by spaces and then fed into std:Command function differently for each event. I wonder if there is a way to make Structopto do this instead?