tictactoe_widget: Convert tictactoe_data_holder to an ES6 class.
tictactoe_widget: Convert tictactoe_data_holder to an ES6 class.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

showell/zulip,zulip/zulip,zulip/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,showell/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,showell/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,andersk/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,rht/zulip,rht/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,zulip/zulip,andersk/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,showell/zulip,andersk/zulip,brainwane/zulip