Build against ltk 0.3.0: add the mandatory App methods to the example and the crate doctest, declare libltk-dev as a build dependency
ltk 0.3.0 makes `app_id`, `save_state` and `restore_state` mandatory on the `App` trait. The `render_in_window` example and the `impl App` snippet in the crate-level docs gain the three methods as stubs (`net.liberux.ltk-webkit.demo`, nothing persisted) so `cargo build --examples` and `cargo test --doc` compile again; the crate itself implements no `App`.
The `ltk` registry path moves to `ltk-0.3.0`, and `debian/control` now lists `libltk-dev (>= 0.3.0), (<< 0.4)` in Build-Depends — the package builds against ltk yet only declared it as a runtime dependency of `libltk-webkit-dev`, so a clean build environment could not resolve it, and with the trait now breaking, the version bound matters.
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# Cambios a ser confirmados:
#	modificados:     Cargo.toml
#	modificados:     debian/control
#	modificados:     examples/render_in_window.rs
#	modificados:     src/lib.rs
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