Remove shadows from notification bubble style

Shadows are problematic, the icon component is not rendered when we use
them, we will investigate in the future.

For now, a darker border makes a similar effect.

Issue: #20
Improved notification bubble gradient

Lighter color for half the bubble.

Issue: #20
Don't process events between show and add surface

The problem was that by the time we called addSurface(), the surface
was not attached, damaged and committed and the compositor crashed.

That was because the compositor expected a proper surface but there's
no need for this, se we simplify things and fix the server rather
the client.