Merge: gamnit: scale text, add custom characters and create links with `BMFont`

Merge: gamnit: scale text, add custom characters and create links with `BMFont`

This PR adds more customization options when drawing text using `BMFont`:

* Change the size of the text with a simple attribute `TextSprites::scale`.
* Insert artificial chars to a `BMFont` using the more permissive `BMFontChar`. This way, one can set custom textures for a letter, an emojis or any character from the private use areas.
* Add "links" to the text using Markdown-like syntax (`[my link]` or `[pretty name](internal name)`). Note that the links are not clickable by themselves. However, the client code can access the sprites representing the link to change their style or to get their position. This will work well with a rumored future feature making sprites clickable...

Notice the colored links and the smaller text box in the following screenshot of `fonts_showcase`:
![screenshot from 2017-07-13 22-09-20](https://user-images.githubusercontent.com/208057/28196740-8f8630b6-6820-11e7-8915-1df02b4edee5.png)

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Once again, these new features are not implemented for `TileSetFont`. I'm considering keeping the API of `TileSetFont` but replacing its implementation by a `BMFont` for it to gets all the new shiny features.

Pull-Request: #2527
Reviewed-by: Jean Privat <jean@pryen.org>