New theme system

[takev]

I want widgets to be able to get theme values:

• It needs to be fast as it is used inside the draw functions.
• The theme needs to be able to change at runtime.
• Use a sort of cascading style sheet for themes.
• Have the hierarchy of widgets be like tags in HTML.
• A developer can assign attributes to a widget using a string:
  • Add zero or one identifier names by prefixing with #
  • Add zero or more class names by prefixing with .
  • Add zero or more style-attributes using name=value.
• The CSS theme file has a selector so that the style can change depending on the depth of a widget in the hierarchy.
  • There needs to be a way to reset the count, for example an overlay-widget resets the count
  • Maybe some widgets that are not visible need to be skipped when counting.

widget attribute syntax:

attribute: ( attribute ( ' ' attribute )* )?
selector: identifier | class | style
identifier: '#' [a-zA-Z-][a-zA-Z0-9-]
class: '.' [a-zA-Z-][a-zA-Z0-9-]
style: style-name '=' style-value
style-name: [a-zA-Z-][a-zA-Z0-9-]
style-value: style-size-value | style-color-value | style-string-value

class my_widget : public widget_intf {
   void draw(widget_draw_context const &content) noexcept {
        auto tmp1 = left_margin();
        auto tmp2 = background_color();
   }
};

class widget_intf {
    theme_context _theme_context;

    float left_margin() const noexcept {
       return (_theme_context.left_margin() * _dpi).numeric_value_in(pixel);
    }
};

struct detail::theme_attributes {
    std::string tag; // if .empty() then attribute is not in use.
    std::string id;
    std::vector<std::string> classes;
    attribute_flags left_margin_flags;
    theme_length left_margin;
};

std::vector<detail::theme_attributes> detail::theme_contexts;

class theme_context {
    uint32_t _id;
    ~theme_context() { detail::theme_contexts_delete_id(_id); }
    theme_context() : _id(detail::theme_contexts_new_id()) {}

   auto left_margin(pixel_density dpi) const noexcept {
      return convert_to_pixel(detail::theme_contexts[_id].left_margin, dpi);
   }
};