Use AdwApplication and support dark mode

Use adw::Application to allow for a separate css sheet for dark mode. I am marking as draft until I can compile Fractal Next and properly test this.

There are other classes which use hardcoded colors:

.sidebar .room .notification_count {
  /* ... */
  background-color: #555;
  color: white;
}

button.cutout {
  box-shadow: 0 0 0 2px @theme_bg_color;
  background-color: #e2e2e1;
}

button.cutout:hover {
  background-color: #d8d8d7;
}

button.cutout:active {
  background-color: #c4c5c4;
}