ui/tui: highlighting refactor

Make HlAttr contain highlighting state for both color modes (cterm and rgb).
This allows us to implement termguicolors completely in the TUI.

Simplify some logic duplicated between ui.c and screen.c. Also avoid
some superfluous highlighting reset events.

test/functional/ui/screen.lua

@@ -455,6 +455,9 @@ function Screen:_handle_visual_bell()
   self.visual_bell = true
 end
 
+function Screen:_handle_default_colors_set()
+end
+
 function Screen:_handle_update_fg(fg)
   self._fg = fg
 end