Implement translation support

[AJenbo]

This brings translation support to the game, it will probably need a bit of adjustment on different platforms and for the handling of static texts. Currently, also only the strings from DiabloUI and appfat.cpp are marked as translatable.

Fixes #66

[AJenbo]

@glebm how do you feel about the way I'm handling the translation of static texts here? I saw it used in other projects, all strings are wrapped in a fake translation function (N_()) and the actual translation then happens when the texts are scheduled for rendering: 1acd179

[glebm]

@AJenbo There is a danger with that approach when mixing N_ and _ calls, described here: https://www.gnu.org/software/gettext/manual/html_node/Special-cases.html

For UI components, perhaps we could mark it explicitly with a wrapper type:

struct UiStringView {
  constexpr UiStringView(const char *text, bool localize = false) 
    : localize_(localize), text_(text) {
  }

  const char *localized() const {
    if (localize_) return gettext(text_);
    return text_;
  }
  // (or make the fields public and move the logic to render/display code)

 private:
  bool localize_;
  const char *text_;
};

The macro would return this type and the static arrays would change from:

-const char *const CREDITS_LINES[] = {
+const UiStringView CREDITS_LINES[] = {

The type above is implicitly constructible from a C-string literal.

[glebm]

E.g. like this: https://godbolt.org/z/4fnjQy

[AJenbo]

If any one can getting this building on Windows, Mac and Swtich I would appreciate the help.

[ghost]

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[AJenbo]

Looks like https://github.com/SuperTux/supertux is a good example of how to get this implemented for Windows and Mac

[AJenbo]

Suggested change

	msgstr "ingen gateway"
	msgstr "gateway behøves ikke"

(check if this fits first)