e_xdg_shell_v6: change commit handler event to surface commit hook

e_xdg_shell_v6: change commit handler event to surface commit hook

Change the target event of the surface commit handler from E_EVENT_CLIENT_BUFFER_CHANGE to E_COMP_WL_HOOK_CLIENT_SURFACE_COMMIT.
Previously, because more weight was placed on the buffer, even when the surface commit came,
the xdg surface information was processed only when the buffer attach had occurred before.

However, according to the spec, it is supposed to be processed when a commit comes.
To match this, the target event has been changed to hook that E_COMP_WL_HOOK_CLIENT_SURFACE_COMMIT, which is generated when a commit comes.

Change-Id: I515802229fabfa8ba3a84c70b457e16f1b767ed2