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