GUI: Allow user to adjust display font size

As a scientist, I want to be able to adjust the size of font used in the GUI, so that I can view the screen from a distance.

'''Acceptance Criteria'''
1. I can grow/shrink the font size.
1. I can reset the font size to its default value.

'''Notes'''
1. Should the user be able to choose font size on a view-by-view basis?
1. Should the user be able to choose the font as well as the font size?
1. Could just use grow/shrink font buttons (see, for example, MS-Word).
1. Some views use larger/smaller fonts than others (e.g. Dashboard vs Synoptic display). If growing fonts globally, should we maintain the relative size of these fonts?

No files or tests are named. Start by locating how the GUI defines font sizes, then compare the Dashboard and Synoptic display views. Done means users can grow, shrink, and reset the font size while preserving appropriate relative sizing across views; the notes require design decisions about scope, font choice, and controls.

Scritto dal modello di indicizzazione a partire dal testo della issue.