use GSettings app instead

Gsettings needs to be freed

Fixes this warning running inside other terminal when closing the window if this preference is enabled: [when command exits "exit the terminal"]

** (mate-terminal:45473): CRITICAL **: 01:32:55.425: terminal_window_remove_screen: assertion 'gtk_widget_get_toplevel (GTK_WIDGET (screen)) == GTK_WIDGET (window)' failed

Fixes #314

terminal-app.c:152:26: warning: passing argument 2 of ‘terminal_app_profile_list_notify_cb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
terminal-app.c:153:29: warning: passing argument 2 of ‘terminal_app_default_profile_notify_cb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
terminal-app.c:155:27: warning: passing argument 2 of ‘terminal_app_encoding_list_notify_cb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
terminal-app.c:143:28: warning: passing argument 2 of ‘terminal_app_system_font_notify_cb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
terminal-app.c:149:35: warning: passing argument 2 of ‘terminal_app_enable_menu_accels_notify_cb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
terminal-app.c:146:30: warning: passing argument 2 of ‘terminal_app_enable_mnemonics_notify_cb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]