Implement complex OO UI

[seclerp]

Add ability to create UI using OxyEngine.

Diagram

Widget structure

Layout widgets:

• HorizontalLayout
• VerticalLayout
• GridLayout
• PixelLayout

Visual widgets:

• Text
• Image
• Button
• InputBox
• InputArea
• CheckBox
• RadioButton
• Slider
• ProgressBar
• ScrollBar

Also introduce EventSystem for controls:

Keyboard:

• keypress - keyboard key is changing state to 'down'
• keyrelease - keyboard key is changing state to 'up'
• keydown - fires while keyboard key is in down state

Mouse:

• mousepress - mouse button is changing state to 'down'
• mouserelease - mouse button is changing state to 'up'
• mousedown - fires while mouse button is in 'down' state
• mousemove - fires if mouse pointer moves inside widget's Rectangle.
• mouseenter - fires if mouse pointer enters widget's Rectangle.
• mouseexit - fires if mouse pointer exits widget's Rectangle.

[seclerp]

Task going to backlog, because of complex implementation