[core] use Unity indicator-keyboard icons in Unity

In Unity, there are icons available for keyboard layouts that Fcitx could use.
If `INDICATOR_KEYBOARD_LENGTH` is using `strlen` and you don't want VLA this can be
```
    char layout[] = INDICATOR_KEYBOARD_PREFIX "Xx";
```
which can also save you the `...[... + 2] = '\0'` below.

Neither of these is big issue though. =)