Extended dark mode and light/dark toggle support

[vladimiry]

Since the app already uses Electron v7 we could utilize the goodies provided by https://www.electronjs.org/docs/api/native-theme API (available since v7).

[vladimiry]

Here is how it's going to look. The default value is "follow system" which means following the system preference.

Only the app UI gets affected. Maybe in new releases the app will apply the mode to @ProtonMail UI too (only applying on the fly without modifying the account dark/light mode preferences).