The render function now takes a self_rc and returns a enum that permits
the implementation to handle rendering of children on its own and
thus make the caller skip that traversal step.
* 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
The README.md contains the warning that used to be in lib.rs.
Add README.md files to all internal crates
... pointing to the official public crate to use instead.
Rename internal crates
fixup: README files
fixup rename
