Allow converting a brush to a color. In the case of a gradient, the color of the first stop is returned.
For the C++ generator this requires adding the extra case of explicitly
calling the `Brush(const Color &)` constructor, despite it being implicit,
in order to generate the correct code when we have IR that casts twice:
```
Expression::Cast {
from: Expression::Cast {
from: Expression::Cast {
from: Expression::NumberLiteral(...),
to: Type::Color,
}
to: Type::Brush,
},
to: Type::Color,
}
```
Add support for built-in property aliases and rename `color` to
`background` - in preparation for it also changing to type brush.
Right now the alias is silent, a deprecation and overall change
will come in a subsequent change.
This also makes the focus() method available as a member function on any
item, but the resolve_element_reference_in_set_focus_calls() pass will
check if the elements are valid.
The check for `has-focus` to determine a focusable item was replaced
with an annotation on the built-in elements, so that `has-focus` can
later be implemented as a built-in function through the run-time,
without the need for a boolean property.
