Situations where a view's width changes:
- changing screen modes
- enter staging or patch building
- resizing the terminal window
For the first of these we currently have special code to force a rerender, since
some views render different content depending on whether they are in full-screen
mode. We'll be able to remove that code now, since this new generic mechanism
takes care of that too.
But we will need this more general mechanism for cases where views truncate
their content to the view width; we'll add one example for that later in this
branch.
This begins a big refactor of moving more code out of the Gui struct into contexts, controllers, and helpers. We also move some code into structs in the
gui package purely for the sake of better encapsulation
