Normalize the headwords stored in the index

Here's the problem:

Let's create the following dictionary:
```
#NAME	"non normalized headwords"
#INDEX_LANGUAGE	"English"
#CONTENTS_LANGUAGE	"English"

cuté
  Single letter.

cuté
  Combining mark.
```

Currently, when user types "cute" in the search field, he will get TWO suggestions, since GD currently considers unicode-equivalent strings as completely different.

With the suggested patch, GoldenDict will just show a single suggestion. But that would require full re-index of all the dictionaries...