Improve app config

[crizzitello]

• Add a defaultValue method to AppConfig and set several defauts
• Fixes Automatically determine linux screen capture application #222 by providing defaults for a few common screen shot tools on linux

[JoelAtDeluxe]

I'm not sure if this is accurate anymore. Gnome changed their screenshot app to pop up a UI. I don't know if the CLI works the same anymore (or, really, at all)

[crizzitello]

It might not be current but it will work for those with gnome-screenshot installed,

Ill see if there is another newer tool for gnome to check for, Oh its now a part of the shell not a standalone application. perhaps we can recommend a tool for those users. and ill add it to the list.

[JoelAtDeluxe]

Actually, yes, that's true. I thought they replaced the binary but it looks like it's not called gnome-screenshot anymore.

Edit: The new screenshotting functionality is apparently built-in to the gnome shell. It doesn't appear that there's a way to invoke it from the terminal.

[JoelAtDeluxe]

The changes look fine to me. A couple of thoughts:

1. Probably not the defaults i'd use, but they're reasonable enough, and obviously you need to pick something.
2. There's a bit of repetition in the code that could be turned into a small lambda -- that's what I'd do in a different language (I'll show below what I'm thinking)
3. I haven't programmed in C++ in awhile, so take everything with a grain of salt.

  if (appConfig->value(CONFIG::SHORTCUT_SCREENSHOT).isNull())
    appConfig->setValue(CONFIG::SHORTCUT_SCREENSHOT, defaultValue(CONFIG::SHORTCUT_SCREENSHOT));

  //why not:
  auto setDefaultIfNull = [appConfig](QString field){  // is it a QString? This is how lazy I am: I can't be bothered to look right now.
    if (appConfig->value(field).isNull())
      appConfig->setValue(field, defaultValue(field));
  }
  setDefaultIfNull(CONFIG::SHORTCUT_SCREENSHOT);
  setDefaultIfNull(CONFIG::I_FORGET_THE_OTHER_FIELDS);

[crizzitello]

 auto setDefaultIfNull = [appConfig](QString field){  // is it a QString? This is how lazy I am: I can't be bothered to look right now.
    if (appConfig->value(field).isNull())
      appConfig->setValue(field, defaultValue(field));
  }

Fixed it we don't even need to see the config if we try to read and it's empty we just get the default due to the change in the value method . Good call :D