Add Blur API

[lenemter]

Fixes #1998

To do:

• Figure out how to handle power saving (probably Granite and applications should handle that??)
• Test with multiple monitors (doesn't make sense yet)
• Test with display scaling
• Fix X11 bug when window opens (probably will be fixed by Use pre configure mechanism for window positioning #2216)
• Test with fractional scaling

[danirabbit]

I'm sure @leolost2605 has some more informed opinions about the UI, but from a design perspective I think it looks clean and is very legible :)

[leolost2605]

This looks very good thank you!
I wonder though if we still want the left, right, top, bottom offsets or if we'd rather want to set the correct shadow size in gtk like it's done in the dock since elementary/dock#437 ?

[lenemter]

@leolost2605 I initially tried this approach, but the problem is dock's bottom margin. If we set it as 'shadow' then there will be no margin. But if we set it as window content then the bottom margin will be blurred.

[leolost2605]

@leolost2605 I initially tried this approach, but the problem is dock's bottom margin. If we set it as 'shadow' then there will be no margin. But if we set it as window content then the bottom margin will be blurred.

Ah I guess we might want a margin property for the shell clients then similar to what wlr layer shell has.
But since it's private API anyways we can always change it later so LGTM :)

[danirabbit]

@lenemter are you happy to merge this?

[lenemter]

@danirabbit I'm happy with the gala part but not so much with the dock. We basically need to either hardcode bottom margin size or allow changing it via gsetting. There's probably a way to achieve this with clever CSS but I'm not ready to dig into it yet.