Issue #1668 - Part 1: Implement support for caret-color property.

This CSS property allows input carets (that blinking input cursor you see in text fields), to be given a custom color. This was implemented in Firefox 53, and it was such a minor feature that no one ever missed it, but I don't see any harm in implementing this.

https://bugzilla.mozilla.org/show_bug.cgi?id=1063162

layout/generic/nsFrame.cpp

@@ -1831,8 +1831,7 @@ nsIFrame::DisplayCaret(nsDisplayListBuilder* aBuilder,
 nscolor
 nsIFrame::GetCaretColorAt(int32_t aOffset)
 {
-  // Use text color.
-  return StyleColor()->mColor;
+  return StyleColor()->CalcComplexColor(StyleUserInterface()->mCaretColor);
 }
 
 bool