shiny/widget: rename the Image widget to Uniform.

A future commit will re-introduce the Image widget, with source-rect, mask and
mask-point properties as per draw.Draw arguments. But that complexity isn't
needed when all you want is a uniform block of color.

Also measure a Uniform widget's size in unit.Value, not necessarily pixels.

Change-Id: Ia9d7a88f42ff8316d5e1febf7814521637595572
Reviewed-on: https://go-review.googlesource.com/21858
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
3 files changed