Set and update terminal color palette

[marceline-cramer]

The API to set colors shouldn't use any alacritty_terminal types.

It may be necessary to expand the arguments to Terminal::new() into a full struct with top-level fields for TerminalSpace, fonts, alpha, and colors, for instance.

I'm not sure how this interacts with writeback from the terminal contents in render. Experimentation may be needed as to if the writeback is even necessary.