Fix issue #32 - separate the keyboard shortcuts in some config file

[sebojanko]

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Hi, I tried to separate the shortcuts to a JSON file. The JSON looks like this:

[
    ["^N", "sort_by_memory", "Memory Sort"],
    ["^T", "sort_by_time", "Time Sort"],
    ["^K", "kill_process", "Kill"],
    ["^Q", "quit", "Quit"],
    ["^R", "reset", "Reset"],
    ["^L", "show_detailed_process_info", "Process Info"],
    ["^F", "do_process_filtering_work", "Filter"],
    ["g", "", "Top"]
]

The first element is the shortcut, the second is the name of the method that is called and third is the name showed to the user. I know letting the user write which method should be called should be avoided but I haven't found any other way to implement this.

Another thing, I used a global variable to save the shortcut key and its name between classes. I've seen you using global variables so I think it should be alright to you. Another way to do this, without using a global variable, is to read the file twice. Let me know what you think about this.

This is my first PR ever to an OS project, so I apologize if I did something wrong. Thanks :)

[darxtrix]

Thanks, let me get back to you after reviewing this !