Add function to get page labels

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>

zathura/page.c

@@ -359,3 +359,34 @@ zathura_page_render(zathura_page_t* page, cairo_t* cairo, bool printing)
 
   return functions->page_render_cairo(page, page->data, cairo, printing);
 }
+
+char*
+zathura_page_get_label(zathura_page_t* page, zathura_error_t* error)
+{
+  if (page == NULL || page->document == NULL) {
+    if (error) {
+      *error = ZATHURA_ERROR_INVALID_ARGUMENTS;
+    }
+    return NULL;
+  }
+
+  zathura_plugin_t* plugin = zathura_document_get_plugin(page->document);
+  zathura_plugin_functions_t* functions = zathura_plugin_get_functions(plugin);
+  if (functions->page_get_label == NULL) {
+    if (error) {
+      *error = ZATHURA_ERROR_NOT_IMPLEMENTED;
+    }
+    return NULL;
+  }
+
+  char* ret = NULL;
+  zathura_error_t e = functions->page_get_label(page, page->data, &ret);
+  if (e != ZATHURA_ERROR_OK) {
+    if (error) {
+      *error = e;
+    }
+    return NULL;
+  }
+
+  return ret;
+}

zathura/page.h

@@ -203,4 +203,15 @@ char* zathura_page_get_text(zathura_page_t* page, zathura_rectangle_t rectangle,
  */
 zathura_error_t zathura_page_render(zathura_page_t* page, cairo_t* cairo, bool printing);
 
+/**
+ * Get page label. Note that the page label might not exist, in this case NULL
+ * is returned.
+ *
+ * @param page Page
+ * @param error Set to an error value (see \ref zathura_Error_t) if an error
+ *    occurred.
+ * @return Page label
+ */
+char* zathura_page_get_label(zathura_page_t* page, zathura_error_t* error);
+
 #endif // PAGE_H

zathura/plugin-api.h

@@ -91,10 +91,15 @@ typedef char* (*zathura_plugin_page_get_text_t)(zathura_page_t* page, void* data
 typedef zathura_image_buffer_t* (*zathura_plugin_page_render_t)(zathura_page_t* page, void* data, zathura_error_t* error);
 
 /**
- * Renders the page
+ * Renders the page to a cairo surface.
  */
 typedef zathura_error_t (*zathura_plugin_page_render_cairo_t)(zathura_page_t* page, void* data, cairo_t* cairo, bool printing);
 
+/**
+ * Get page label.
+ */
+typedef zathura_error_t (*zathura_plugin_page_get_label_t)(zathura_page_t* page, void* data, char** label);
+
 
 struct zathura_plugin_functions_s
 {
@@ -179,9 +184,14 @@ struct zathura_plugin_functions_s
   zathura_plugin_page_render_t page_render;
 
   /**
-   * Renders the page
+   * Renders the page to a cairo surface.
    */
   zathura_plugin_page_render_cairo_t page_render_cairo;
+
+  /**
+   * Get page label.
+   */
+  zathura_plugin_page_get_label_t page_get_label;
 };
 
 /**