ecore_wl2_window: added new aux_hint_generate API

ecore_wl2_window: added new aux_hint_generate API

We added new APIs for handling the aux_hint as below.

API added
 - int ecore_wl2_window_aux_hint_generate(Ecore_Wl2_Window *win, const char *hint, const char *val);
 - int ecore_wl2_window_aux_hint_id_get(Ecore_Wl2_Window *win, const char *hint);
 - const char *ecore_wl2_window_aux_hint_value_get(Ecore_Wl2_Window *win, int id);

The ecore_wl2_window_aux_hint_generate works similar as ecore_wl2_window_aux_hint_add,
however, this API is no need to give an ID.
It is assigned automatically in this function.

The ecore_wl2_window_aux_hint_id_get returns hint ID corresponding to the "hint" parameter.

The ecore_wl2_window_aux_hint_val_get returns hint value corresponding to the "id" parameter.

Change-Id: I3bec6844c6f2ce7d5105c5792961cb8bae144480
Signed-off-by: Junseok, Kim <juns.kim@samsung.com>