Bug 1929460 - Use .labelColor for caption and dialog/window colors. r=mac-reviewers,spohl

As per [1], this is probably a better generic color, specially when atop
translucent effects / NSVisualEffectView:

> Instead of defining custom grayscale color assets, consider using the
> built-in colors labelColor, secondaryLabelColor, tertiaryLabelColor,
> and quaternaryLabelColor. While typically used with text, these colors
> are applicable with any app content. The built-in colors represent
> varying levels of contrast for your content, with labelColor offering
> the most contrast, and quaternaryLabelColor offering the least
> contrast.

In practical terms for default configurations, it means that the default
light mode color becomes semi-transparent (0, 0, 0, .87) in Sonoma,
which looks a bit less harsh against white backgrounds, which UX brought
up over Slack. In dark mode colors are the same.

I've also verified this looks good with increase-contrast mode etc.

[1]: https://developer.apple.com/documentation/appkit/nsvisualeffectview#1674177

Differential Revision: https://phabricator.services.mozilla.com/D228115

widget/cocoa/nsLookAndFeel.mm

@@ -225,10 +225,6 @@ nsresult nsLookAndFeel::NativeGetColor(ColorID aID, ColorScheme aScheme,
                    ? GetColorFromNSColor(NSColor.controlTextColor)
                    : NS_RGB(0xFF, 0xFF, 0xFF);
       break;
-    case ColorID::Windowtext:
-    case ColorID::MozDialogtext:
-      aColor = GetColorFromNSColor(NSColor.windowFrameTextColor);
-      break;
     case ColorID::Appworkspace:
       aColor = NS_RGB(0xFF, 0xFF, 0xFF);
       break;
@@ -344,7 +340,9 @@ nsresult nsLookAndFeel::NativeGetColor(ColorID aID, ColorScheme aScheme,
     case ColorID::MozHeaderbarinactivetext:
     case ColorID::Inactivecaptiontext:
     case ColorID::Captiontext:
-      aColor = GetColorFromNSColor(NSColor.textColor);
+    case ColorID::Windowtext:
+    case ColorID::MozDialogtext:
+      aColor = GetColorFromNSColor(NSColor.labelColor);
       return NS_OK;
     case ColorID::MozHeaderbar:
     case ColorID::MozHeaderbarinactive: