Add a modal Drawer component

[afarber]

Add a Drawer component to PixiUI for sliding panels that attach to screen edges:

• Support four positions: bottom, top, left, right
• Automatically span full screen width (top/bottom) or height (left/right)
• Support backdrop with closeOnBackdropClick to dismiss on tap
• Enable swipe-to-close gestures
• Accept Graphics, Sprites, Textures, or texture names via GetViewSettings
• Support NineSliceSprite backgrounds for scalable borders
• Emit onClose signal when drawer closes
• Include open/close animations using tweedle.js