[wip] start implementation of OrbRenderContext2D.

examples/widgets.rs

src/application/window.rs

src/core/mod.rs

src/core/orbital/backend.rs

src/core/orbital/mod.rs

src/core/orbital/render_context_2d.rs

src/core/render_context_2d/brush.rs

src/core/render_context_2d/font.rs

src/core/render_context_2d/image_element.rs

src/core/render_context_2d/mod.rs

src/layout/font_icon_size.rs

src/layout/text_selection_layout.rs

src/layout/text_size.rs

src/lib.rs

src/render_object/font_icon.rs

src/render_object/image.rs

src/render_object/mod.rs

src/render_object/rectangle.rs

src/render_object/text.rs

src/systems/event_system.rs