perf: cursor move events only redraw when changed

[tbung]

This PR adds a new function to handle CursorMoved and WinScrolled. On CursorMoved it only rehighlights when the cursor left the cell, WinScrolled only redraws the output if the end of the cell is in the visible range of the window.

This closes #40 .

[dccsillag]

I don't get the purpose of this conditional, and it doesn't seem right to me.

self.nvim.funcs.line("w$") will give the line number of the last visible line in the window (note: this is a Vim API and line numbers are different in neovim APIs; that's why it's < and not <=, I imagine). But what if part of the cell is visible but its end is past the end of the buffer?

Furthermore, why is this done:

                if self.display_window is not None: # and self.nvim.funcs.winbufnr(self.display_window) != -1:
                    self.nvim.funcs.nvim_win_close(self.display_window, True)
                    self.canvas.clear()
                    self.display_window = None
                self._show_outputs(self.outputs[selected_cell], selected_cell.end)
                self.canvas.present()

instead of just

self.clear_interface()
self._show_outputs(self.outputs[selected_cell], selected_cell.end)
self.canvas.present()

?