Compatibility breakage

[pravic]

• [ ] sciter::Value: remove indexing (works via a hack now)
• [ ] sciter::Value::isolate should be unsafe
• [x] s2w : return a single value only, s2wn returns value and length
• [x] window::with_size and K°: use window::Builder instead.
• [ ] window size as array instead of tuple: [width, height] (why? totally forgot now)
• [ ] window creation as result
• [ ] proper error type (probably will break return types) - though our return types are compatible with std::error::Error now
• [x] Window::load_html and Window::load_file should return bool (even better - a proper Result)
• [ ] Window::load_html and Window::load_file should return a proper Result
• [ ] Window::load_file should accept AsPath trait
• [ ] dom::Element:: get_value, get_hwnd, attribute, get_style_attribute should return result
• [ ] dom::Element:: from_window, from_point, from_focus, from_highlighted should return Result<Option<Element>>
• [ ] dom::Element:: get_tag, get_text should return a proper result
• [ ] replace enums with associated constants #92