Add image control

[roblillack]

This PR adds a simple, controlled ui.Image component which can be used to show any kind static or dynamic bitmap data:

&ui.Image{
    X: 10, Y: 10, Width: 300, Height: 300,
    Image: /* a Go image.Image */,
    OnClick: func(x, y int, secondary bool) {
        // ... where was the image clicked? ...
    },
},

Limitations:

• OnClick callback is not particularly flexible:
  • position is reported in the control's coordinate system, not the source image (which might be scaled)
  • no support for dragging the mouse while it is clicked