Canvas and ImageViewer now provide consistent completed-tap activation

Canvas gains a single Pressed(x, y) event alongside DoublePressed(x, y), so custom-drawn controls can respond to a completed mouse, touch, or pen tap without manually combining low-level mouse events. ImageViewer uses the same completed-tap semantics for its existing Pressed event. Published guidance explains custom-drawn button states, SVG/vector assets, theme changes, focus, and keyboard activation.