* Build the backends without rtti unless needed
Only the interpreter needs the rtti generated code.
* Since `SlintElement` emits `#[cfg(feature = "rtti")]` tokens, each
crate using `SlintElement` needs to have an `rtti` feature.
* The selector gets backend specific rtti selection features, in order
for the interpreter to enable them.
* Enable rtti when building the C++ project with the interpreter enabled
With the help of dark-light crate this is now exposed as dark-style
boolean property in NativeStyleMetrics.
Co-authored-by: Olivier Goffart <olivier@slint-ui.com>
Co-authored-by: Simon Hausmann <simon.hausmann@slint-ui.com>
