Add option for scale interface

[Psayker]

Add to preferences option to select intrerface scale

[giantplaceholder]

@Psayker I don't think that everything is being scaled properly. For instance, nicks in the contact list seem to stay the same regardless of the option selected.

[eerielili]

Just my two cents, as in dino#1495, you can do a

      Util.force_css(window, "* { font-size: " + settings.zoom_level.to_string() + "% ;} " ); 

(source: https://github.com/dino/dino/blob/1c4d47f35274d560ee9eb855d7f1c45dd4dd140c/main/src/ui/application.vala#L116-L121)

in application.vala to scale all text in the chat list panel and message panel. Buttons and other UI elements are another story, I didn't implement it in my PR.