This way, when the user changes their color theme, any sheets
that did not have a specific color set will change with the theme.
Also add a button to clear color overrides from a sheet, because
any schematics that have been saved before this change will have
the colors set to something other than UNSPECIFIED.
This introduces COLOR_SWATCH, which is a reusable
widget that shows a color swatch and can invoke the colour picker
when duble/middle clicked.
It uses it's own wxCommandEvent to signal the change.
This makes the layer widget simpler internally, and also allows other
code to show identical swatches if needed.
Jon Evans
Jeff Young
Ian McInerney
jean-pierre charras
John Beard