Multiple component in the interpreter

[ogoffart]

Multiple component export is implemented in Rust and C++ to open different window. But the interpreter still can only generate one componenet and so only one component can be exposed to JS and Python at the moment.

We need to

• Deprecate the slint_interpreter::ComponentCompiler, and replace it by a Compiler

// (pseudo-code)

pub struct CompilationResult {
    components: HashMap<String, ComponentDefinition>,
    diagnostics: BuildDiagnostics,
}

pub impl Compiler {
    // all the options setters

    pub fn build_from_path(...) -> CompilationResult
}

impl CompilationResult {
    pub fn has_error(&self) -> bool
    /// only exported ones
    pub fn component_names(&self) -> impl Iterator<Output = String>

    /// Will lazily do the later passes and may report further errors.
    /// Can also be used on non-exported component? (for the preview)
    pub fn component(&str) -> Option<ComponentDefinition>
    pub fn diagnostics(&self) -> ...
}

• Adapt JS and python to support mutiple components.

• Adapt the LSP's live preview to generate a particular component instead of using the hack. We might still want to tell the Compiler to only generate one particular component to be faster.