Prevent computer from suspending while task is running

[Philip-Scott]

You start up a long command, and you go for a cup of coffee. When you come back you realized that your computer went to sleep while during this time, and even the long command you ran has timed out. Terminal should be smart enough to prevent your computer from suspending while this is running.

Videos does it with this code: https://github.com/elementary/videos/blob/master/src/Services/Inhibitor.vala, but if we're also going to add this feature, maybe we should add it to Granite to keep all the inhibiting code in one place as well as opening it to app developers (Spice-Up uses the same code to prevent the computer from suspending while presenting)