String encapsulation for improved translation

Translating the variables surely is an interesting proposition, but I wonder if it might not reintroduce some of the issues we identified with `%d`? More concretely, should translators be expected to keep track of the fact that the translation of `"total pages"` should be character-identical to the variable string in `"page {current page} of {total pages}"`? Perhaps that could be somewhat alleviated by using `"{total pages}"` in both places, like this:

```lua
self.page_info_text.text = util.template{ 
    _("page {current page} of {total pages}"),
    [_("{current page}" )] = self.page,
    [_("{total pages}")] = self.page_num
}
```

> And we may need to add an note on the tutorial of the template system that the braces around the placeholder should be half width "{}" instead of full width "｛｝"

I wouldn't even know how to *type* a full-width curly bracket and thanks to the `Compose` key I can easily type quite a few characters. I thought that was more of an automatic word-spacing thing that e.g. (La)TeX would apply automatically to display formulas nicely. ;)