Start unifying schema types

Schemas per se are type agnostic, isLookup property is removed. Data is stored and accessed differently based on how it is entered and retrieved.

The crucial change is introduction of the composite key of pid and rid. Previous page data utilizes rid = 0 to differentiate itself. Other types, notably lookup, have autoincrementing rid.

Database migration is not implemented yet.

admin/assignments.php

@@ -137,7 +137,7 @@ class admin_plugin_struct_assignments extends DokuWiki_Admin_Plugin {
         echo '<td><input type="text" name="assignment[assign]" /></td>';
         echo '<td>';
         echo '<select name="assignment[tbl]">';
-        foreach(Schema::getAll('page') as $table) {
+        foreach(Schema::getAll() as $table) {
             echo '<option value="' . hsc($table) . '">' . hsc($table) . '</option>';
         }
         echo '</select>';