Generalized defaults/overrides for notification properties

Ref https://github.com/emersion/mako/pull/559
Ref https://github.com/emersion/mako/issues/211

Proposal: `override.*` keys to override properties, combined with generalized criteria to select when to override.

```ini
[!icon]
override.icon=default

[timeout>1000]
override.timeout=1000
```

Overriding interacts a bit weirdly with criteria. Do we perform another criteria pass just to apply overrides?