Merge lp://staging/~mterry/unity8/less-ubuntucolors into lp://staging/unity8

Change some dash button colors from orange to green.

And switch other colors from hardcoded values to the same semantic theme value.

UbuntuColors should be avoided when possible. Instead we should use semantic color names from the theme. These are future-proof and potentially locale-proof (for locales where green and red mean different things -- which I heard was a thing, but can't find anything on it in a quick search).

Half this branch simply replaces some color names with the correct theme color. So no visual change there. But it also changes some colors:

plugins/Dash/AudioProgressBar.qml: progress bar color from orange to "activity" (blue)
plugins/Dash/ScopeStyle.qml: preview button color from orange to "positive" (green)
qml/Dash/DashPageHeader.qml: active filter color from orange to "positive" (green)
qml/Dash/Previews/PreviewActions.qml: button color from orange to "positive" (green)
qml/Dash/Previews/PreviewPayments.qml: purchase button color from orange to "positive" (green)

I didn't change any uses of the various "grey" colors (slate, jet, charcoal, etc). That seemed a bit more fiddly and less user visible. But they should in theory be fixed down the road.

 * Are there any related MPs required for this MP to build/function as expected? Please list.
 Just the pre-req (lp:~cimi/unity8/uitkPageHeader)

 * Did you perform an exploratory manual test run of your code change and any related functionality?
 Not yet.. will do in a bit

 * If you changed the packaging (debian), did you subscribe the ubuntu-unity team to this MP?
 NA

 * If you changed the UI, has there been a design review?
 No... Do we need one for just using the correct theme colors? The only border case I'd see is the purchase button -- maybe that's supposed to be branding-colored?