Windows flicker when opening

[araujoarthur0]

Whenever we open a new BrowserWindow in the app (waiver manager, preferences) it will flicker for a while with a white background while the elements show and the colors change.

We can make use of electron's 'ready-to-show' event to delay showing only once the window is properly drawn, hiding away the "ugliness" of it becoming ready.
https://www.electronjs.org/docs/latest/api/browser-window#using-the-ready-to-show-event