Component docking, padding and margin system

[Syncaidius]

Add support for various UI alignment features:

• [x] Docking/anchoring - Allow a UIElement to be docked or anchors to one or more edges of it's parent render bounds
• [x] Padding - Allow a UIElement to apply padding, to move child elements further inward, away from the edges of its render bounds
• [x] Margins - ALlow a UIElement to set its own margins, which move it away from one or more edges of its parent