Per https://www.electron.build/icons, a single 512x512 icon.png placed in the
buildResources directory (default `build`) is enough to generate icons for all
three - macOS, Windows and Linux.
...hacky because it was using a timer instead of the "ready-to-show" event
(which wasn't getting fired for some reason). It is recommended that we set a
background color anyway.
We now bundle the renderer code within the app. So a load-fail indicates
something really wrong, not something we can deal with upfront (the code wasn't
probably even working - e.g. it was assigning to a function parameter
mainWindow, not the actual global var behind it).
