feat(neon_framework): Support custom backgrounds

[provokateurin]

Closes #939

I'm pretty sure this whole feature but also the way I implemented it is going to be quite controversial, but please go ahead reviewing :)

[Leptopoda]

I haven't looked much at the code but it looks fine at a first glance.
But the design is just awe full and unacceptable.

I really struggle reading any text displayed so that's also a hard no go regarding #300.

[Leptopoda]

I think we should not have a background behind the main app content but i quite like how the nextcloud web interface has the image behind the drawer (with sufficient contrast!!!).
The contrast is somewhat acceptable when both enabling dark mode and the oled override.
As the PR isn't rebased yet so I couldn't have a look at the dashboard app yet.

I think if the option is enabled we should only show the image behind the drawer. The dashboard app could then make its scaffold background transparent.
Each client developer should decide this on their own without a global rule.

[provokateurin]

To implement this it would be great if there is a way to get cropped part of the background image. We don't want the image to repeat itself so all widgets would need to take the part of the image of their position as the background. I don't know if there is an easy way to achieve it, but using transparency like right now is definitely not good because it will break easily.

[Leptopoda]

We can still use the transparency approach but just not set it globally and only in the scaffold of the dashboard client

[provokateurin]

The transparency approach is hard to implement and adds unnecessary code. "Punching a hole" would be way cleaner and easier to use everywhere.

[provokateurin]

For now I changed it so that it is a widget that can be used independently. Only used in the dashboard right now, but we can expand it later.