Add new `single-key-shortcuts` GSetting, Application.Configuration
property and add UI for it in the Preferences window. When enabled,
load in new `single-key-shortcuts.css` file that has the appropriate
bindings. Update bindings to match Google's current set, and update
shortcut help to match.
Move MainWindow out of the root namespace and add it to Application,
so it can privately share internal state with the Controller. Rename
the source and UI files to match.