Lots of new features, new entity, and string optimizations.

• Added function in InputHelper to change mouse position.
• Added source rectangle for images.
• Added background color for icons.
• Fixed dropdown / list selection problem when scaling.
• Optimize strings usage and getting keyboard input to produce less garbage.
• Added shadow scale style property.
• Added button custom skins.
• Added option to change panel and buttons skins after creation.
• Added Identifier to all entities + Find() functionality to locate children by identifier and / or type.
• Added support in size percents (eg size that is certain percent of its parent size).
• Added BringToFront() helper function.
• New entity: PanelTabs.
• Made all entities support Outline styling by default.