widget/material: add disabled state support to all widgets

This commit configures all remaining widgets to draw themselves in a disabled state
when their layout.Context is disabled. A description of the
strategy employed by each follows:

- Checkbox and RadioButton: Draws the icon component in a lighter color. Currently the label text is left
in its default color.
- ProgressBar: The "progress" color is lightened, but not as much as the background color. This makes the current progress value still readable.
- Editor: The cursor is no longer drawn and the text is lightened.
- Switch: The track is unchanged, but the circular "thumb" component is lightened.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/material/progressbar.go

@@ -64,7 +64,11 @@ func (p ProgressBarStyle) Layout(gtx layout.Context) layout.Dimensions {
 		}),
 		layout.Stacked(func(gtx layout.Context) layout.Dimensions {
 			fillWidth := (progressBarWidth / 100) * float32(progress)
-			return shader(fillWidth, p.Color)
+			fillColor := p.Color
+			if gtx.Queue == nil {
+				fillColor = mulAlpha(fillColor, 200)
+			}
+			return shader(fillWidth, fillColor)
 		}),
 	)
 }