[Gallery] Delay the splashscreen based on `FontFamilyHelper.PreloadAsync` completion

[jeromelaban]

Fonts may be loaded completely after the main screen has been displayed, and delaying the splash screen disappearance (Window.Activate) could be a better way to handle this async behavior.

[agneszitte]

@jeromelaban what is the priority of this issue at the moment please (high or medium)?

[jeromelaban]

medium

[agneszitte]

@jeromelaban is this issue still relevant lately with the latest changes for fonts?

[kazo0]

Something we can use ExtendedSplashScreen and Loadable for?

[jeromelaban]

It's still valid yes.

[rajamatt]

@jeromelaban @kazo0 since we're using the fonts from Uno.Material now (#739), is there a way to use FontFamilyHelper.PreloadAsync("uno material fonts") with them?

Also, with the preloading done within the Wasm project, what's the best way to await the preloading right before the window.Activate inside the UWP head?