Code review comment for lp:~mir-team/mir/unify-pointer-button

Added mir_pointer_button_Count...good call...tbh I dont like this solution that much but I'm concerned about the extensibility with flags (e.g. running out of bits). In particular what if devices have buttons only available through some sort of mir_input_device_list_buttons_ API (e.g. they don't correspond to forward/back...)? ON the other hand I guess this device wouldn't be a pointer...maybe flags are fine...I'd like to get another opinion! RAOF?