Allow block configuration to only show 2 tiers of the menu if desired

Problem/Motivation

Responsive Dropdown Menus blocks always show 3 tiers of a menu. We had a business requirement to only show 2 tiers of the menu.

It should also be mentioned that the current use of menu_tree_all_data() passes the default (3) as the second argument, when $max_depth is in fact the third parameter (see function docs). That will also be fixed.

Proposed resolution

Add a configuration option (that still defaults to 3) in the Responsive Dropdown Menus blocks that allows site builders to choose between showing 2 or 3 tiers.

Remaining tasks

Review/test the attached patch and, if it is deemed acceptable, commit it! :)

User interface changes

Responsive Dropdown Menus will now have a configuration option unique to the module.