Bug fix and improved API

Bug fixes and improvements to API.

• Fixed exception when clearing a panel with scrollbars.
• Added click-through feature, to make entities pass events through them.
• Fixed mouse leave and mouse up events when entity becomes invisible / disabled.
• Added UserInterface.TargetEntity that you can use to determine the entity currently pointed on.
• Added option to insert child at a given index.
• Refactor - made entities DefaultSize a static property, so you can access things like Button.DefaultSize without having to instanciate it.
• Fixed out-of-screen buttons in tabs panel that's attached to the top of the screen.
• Added option to easily set per-panel skin in tabs panel.