[Feature] Implement getMetadata() function for DjVu documents (#753)

This implements the getMetadata() function for DjVu documents, following in the style of the same for the PDF implementation.

This just dumbly copies all the metadata key-value pairs into a lua table. The intent is to give the frontend everything we possibly can and let it figure out what to do with the info, an example of which can be found in the companion pull request koreader/koreader#4314.

djvu.c

@@ -162,6 +162,32 @@ static int setColorRendering(lua_State *L) {
 	return 0;
 }
 
+static int getMetadata(lua_State *L) {
+	DjvuDocument *doc = (DjvuDocument*) luaL_checkudata(L, 1, "djvudocument");
+	miniexp_t anno = ddjvu_document_get_anno(doc->doc_ref, 1);
+	miniexp_t *keys = ddjvu_anno_get_metadata_keys(anno);
+
+	// `keys` can be null if there's an error. In that case,
+	// we have the choice of returning either `nil` or
+	// the empty table back to lua.  Here we prefer the latter.
+	lua_newtable(L);
+	if (!keys) return 1;
+
+	int i;
+	for (i = 0; keys[i] != miniexp_nil; i++) {
+		const char *value = ddjvu_anno_get_metadata(anno, keys[i]);
+
+		if (value) {
+			lua_pushstring(L, miniexp_to_name(keys[i]));
+			lua_pushstring(L, value);
+			lua_settable(L, -3);
+		}
+	}
+
+	if (keys) free(keys);
+	return 1;
+}
+
 static int getNumberOfPages(lua_State *L) {
 	DjvuDocument *doc = (DjvuDocument*) luaL_checkudata(L, 1, "djvudocument");
 	lua_pushinteger(L, ddjvu_document_get_pagenum(doc->doc_ref));
@@ -716,6 +742,7 @@ static const struct luaL_Reg djvu_func[] = {
 
 static const struct luaL_Reg djvudocument_meth[] = {
 	{"openPage", openPage},
+	{"getMetadata", getMetadata},
 	{"getPages", getNumberOfPages},
 	{"getToc", getTableOfContent},
 	{"getPageText", getPageText},