[update] api simplify

README.md

examples/canvas.rs

examples/grid.rs

examples/image.rs

examples/light-theme.rs

examples/messages.rs

examples/minimal.rs

examples/widgets.rs

src/application/tree.rs

src/application/window.rs

src/backend/mod.rs

src/backend/orbital/backend.rs

src/backend/orbital/mod.rs

src/backend/orbital/renderer.rs

src/event/event_queue.rs

src/layout/fixed_size.rs

src/layout/mod.rs

src/layout/padding.rs

src/properties/layout/column.rs

src/properties/layout/row.rs