Rewrite symbols tree updating code

Old implementation was not really fitting the updating needs and had a
bug making symbols disappear if they haven't changed but their parent
did (e.g. when a C++ constructor's signature changed).

New implementation does:
1) walk old tree, updating or removing rows;
2) add remaining tags.

It walks less than (new_tags + old_tags + new_tags) in the worst case,
thanks to some hash table-based caching;  and also gets rid of the
"valid" column in the symbols tree, saving a few bytes in memory.

Finally, there is a ~7% performance gain (from 21 to 18ms) upon common
tree updates, sometimes more.

src/sidebar.c

@@ -216,7 +216,7 @@ void sidebar_update_tag_list(GeanyDocument *doc, gboolean update)
 		if (doc->priv->tag_tree == NULL)
 		{
 			doc->priv->tag_store = gtk_tree_store_new(
-				SYMBOLS_N_COLUMNS, GDK_TYPE_PIXBUF, G_TYPE_STRING, TM_TYPE_TAG, G_TYPE_STRING, G_TYPE_BOOLEAN);
+				SYMBOLS_N_COLUMNS, GDK_TYPE_PIXBUF, G_TYPE_STRING, TM_TYPE_TAG, G_TYPE_STRING);
 			doc->priv->tag_tree = gtk_tree_view_new();
 			prepare_taglist(doc->priv->tag_tree, doc->priv->tag_store);
 			gtk_widget_show(doc->priv->tag_tree);

src/sidebar.h

@@ -41,7 +41,6 @@ enum
 	SYMBOLS_COLUMN_NAME,
 	SYMBOLS_COLUMN_TAG,
 	SYMBOLS_COLUMN_TOOLTIP,
-	SYMBOLS_COLUMN_VALID,
 	SYMBOLS_N_COLUMNS
 };