elm_entry: handle cursor delete/backspace with clusters consist of one or multible...

elm_entry: handle cursor delete/backspace with clusters consist of one or multible glyphs

Summary:
Cluster consist of one glyph, expected to be removed on backspace or delete key.
Cluster consist of multible glyph, expectd to remove part of on backspace or delete key.

This is behaviour founded in Android. (our current way of handling similar to Qt)

**New Behaviour**
{F3750386}

**Old Behaviour**
{F3750387}

Test Plan:
Auto Testing is challenging because there are no easy way to emulate keyboard down on elm_entry
```
#include <Elementary.h>

EAPI_MAIN int
elm_main(int argc, char **argv)
{
   Evas_Object *win,*box,*entry;

   elm_policy_set(ELM_POLICY_QUIT, ELM_POLICY_QUIT_LAST_WINDOW_CLOSED);

   win = elm_win_util_standard_add("", "");
   elm_win_autodel_set(win, EINA_TRUE);

   box = elm_box_add(win);
   entry = elm_entry_add(box);

   evas_object_size_hint_weight_set(box,EVAS_HINT_EXPAND,EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(box,EVAS_HINT_FILL,EVAS_HINT_FILL);

   elm_entry_entry_set(entry,"<font=NotoColorEmoji wrap=mixed >🇧🇬อั🇧🇬อั&#x1F600;&#x1F600;&#x1F600;&#x1F600;&#x1F600;อั</font>");
   evas_object_size_hint_weight_set(entry,EVAS_HINT_EXPAND,0.9);
   evas_object_size_hint_align_set(entry,EVAS_HINT_FILL,EVAS_HINT_FILL);
   evas_object_show(entry);
   evas_object_show(box);

   elm_box_pack_end(box,entry);
   elm_win_resize_object_add(win,box);
   evas_object_resize(win,320,480);

   evas_object_size_hint_weight_set(entry,EVAS_HINT_EXPAND,0.1);
   evas_object_size_hint_align_set(entry,EVAS_HINT_FILL,EVAS_HINT_FILL);

   evas_object_show(win);
   elm_run();

   return 0;
}
ELM_MAIN()

```

Reviewers: tasn, woohyun, bowonryu

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9628