Added cffi/* module for third-party advanced modules

resources/custom_modules/cffi_example/waybar_cffi_module.h

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <gtk/gtk.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/// Waybar ABI version. 1 is the latest version
+extern const size_t wbcffi_version;
+
+/// Config key-value pair
+struct wbcffi_config_entry {
+  /// Entry key
+  const char* key;
+  /// Entry value as string. JSON object and arrays are serialized.
+  const char* value;
+};
+
+/// Module init/new function, called on module instantiation
+///
+/// MANDATORY CFFI function
+///
+/// @param root_widget        Root GTK widget instantiated by Waybar
+/// @param config_entries     Flat representation of the module JSON config. The data only available
+///                           during wbcffi_init call.
+/// @param config_entries_len Number of entries in `config_entries`
+///
+/// @return A untyped pointer to module data, NULL if the module failed to load.
+void* wbcffi_init(GtkContainer* root_widget, const struct wbcffi_config_entry* config_entries,
+                  size_t config_entries_len);
+
+/// Module deinit/delete function, called when Waybar is closed or when the module is removed
+///
+/// MANDATORY CFFI function
+///
+/// @param instance Module instance data (as returned by `wbcffi_init`)
+void wbcffi_deinit(void* instance);
+
+/// When Waybar receives a POSIX signal, it forwards the signal to each module, calling this
+/// function
+///
+/// Optional CFFI function
+///
+/// @param instance Module instance data (as returned by `wbcffi_init`)
+/// @param signal Signal ID
+void wbcffi_refresh(void* instance, int signal);
+
+///
+/// Optional CFFI function
+///
+/// @param instance Module instance data (as returned by `wbcffi_init`)
+/// @param name Action name
+void wbcffi_doaction(void* instance, const char* name);
+
+#ifdef __cplusplus
+}
+#endif