ui: forward relevant option updates to UIs (#7520)

also make termguicolors mutable after startup

test/functional/ui/screen.lua

@@ -137,6 +137,7 @@ function Screen.new(width, height)
     visual_bell = false,
     suspended = false,
     mode = 'normal',
+    options = {},
     _default_attr_ids = nil,
     _default_attr_ignore = nil,
     _mouse_enabled = true,
@@ -482,6 +483,10 @@ function Screen:_handle_set_icon(icon)
   self.icon = icon
 end
 
+function Screen:_handle_option_set(name, value)
+  self.options[name] = value
+end
+
 function Screen:_clear_block(top, bot, left, right)
   for i = top, bot do
     self:_clear_row_section(i, left, right)