Add accent color support to QPalette

Platform themes on Windows and macOS define an "accent color" that is used by relevant UI components to highlight certain parts. It's not always the same as the text highlighting (which we already have in QPalette::Highlight and QPalette::HighlightedText), ie. a line edit on Windows might use the accent color to show that the line edit has focus, while using a different color for the text highlighting.

QPalette needs to get a new color role, which comes with a number of challenges:

• we have already used 63 of the 64 bits in the resolve mask (21 color roles, 3 color groups)
• For each color role, we need 3 bits (one for each color group) - so we need 2 more bits
• the ResolveMask is a 64bit value and used in public, but internal API
• new palette color roles need to be supported in Qt style sheets and in the respective Qt Quick APIs