Create multi-level drop down menu sample

[RivaIvanova]

Description

Create a sample that demonstrates a multi-level menu using IgxDropDownComponent.

For example:

• [x] It should mimic the Infragistics website's menu.

• [x] The Design & Development section should demonstrate multi-level dropdowns.
• [x] The nested dropdown should be opened/closed on hover.
• [x] The Web, Desktop, Cross Platform, Design to Code, Automated Testing Tools and Ignite UI categories should not be selectable.
• [x] Selecting an item from any of the nested dropdowns should close all dropdowns.