api: Implement the 'redraw:cursor' event
api: Implement the 'redraw:win_end' event
api: Implement vim_request_screen to force redraw events
api: Implement vim_command_output function

This function can be used by API clients to execute a command and capture the
output.
Use character offsets instead of byte offsets for attributes
Redraw when colorscheme changes
Fix attribute table lookup
api: Implement 'redraw:{start,end}' events