Custom Loading screen

[Ze-Rax]

There's a LayerType CUILayer::EUILayerType::LoadingScreen. Maybe we can construct a nice Manialink as loadingscreen in our color scheme.

Layer would have to be attached at map end, because at that time all layers are destroyed.

Loading screen should have have a loading indicator (like the current spinning wheel) so people know that their game didn't freeze.

I'd be ok with using a custom image there, but then it would need to be hosted in a proper place, wher it always is accessible..