charmap: bug fix

Use only methods that can be called using GI.

The traceback in the terminal was:
```
Traceback (most recent call last):
  File "[...]/gedit/plugins/charmap/panel.py", line 61, in on_chapter_view_selection_changed
    codepoint_list = self.view.get_codepoint_list()
AttributeError: 'ChaptersView' object has no attribute 'get_codepoint_list'
```

Fixes https://gitlab.gnome.org/GNOME/gedit-plugins/-/issues/41