Add prototypes for all plugin symbols

This allows the compilers to check that the plugin's symbols have the
proper prototype.  Doing so can avoid subtle and hard-to-find bugs in
case a plugin's symbol has incorrect signature as dlsym() can't check
if the signature is actually the one we expect.

As a bonus, it helps when using -Wmissing-prototypes as it provides the
prototypes.

src/plugindata.h

@@ -157,15 +157,6 @@ GeanyPlugin;
 	}
 
 
-#ifndef GEANY_PRIVATE
-
-/* Prototypes for building plugins with -Wmissing-prototypes */
-gint plugin_version_check(gint abi_ver);
-void plugin_set_info(PluginInfo *info);
-
-#endif
-
-
 /** @deprecated - use plugin_set_key_group() instead.
  * @see PLUGIN_KEY_GROUP() macro. */
 typedef struct GeanyKeyGroupInfo
@@ -258,6 +249,23 @@ GeanyData;
 #define geany			geany_data	/**< Simple macro for @c geany_data that reduces typing. */
 
 
+#ifndef GEANY_PRIVATE
+
+/* Prototypes for building plugins with -Wmissing-prototypes
+ * Also allows the compiler to check if the signature of the plugin's
+ * symbol properly matches what we expect. */
+gint plugin_version_check(gint abi_ver);
+void plugin_set_info(PluginInfo *info);
+
+void plugin_init(GeanyData *data);
+GtkWidget *plugin_configure(GtkDialog *dialog);
+void plugin_configure_single(GtkWidget *parent);
+void plugin_help(void);
+void plugin_cleanup(void);
+
+#endif
+
+
 /** This contains pointers to functions owned by Geany for plugins to use.
  * Functions from the core can be appended when needed by plugin authors, but may
  * require some changes. */