Implement Inventory Favorites Tab

[akleshchev]

Implement Inventory Favorites Tab according to mockup.
Branch: https://github.com/secondlife/viewer/tree/project/inventory_favorites
See #1300 for details about favorites

[kylelinden]

Work on Inventory Favorites in a branch for reviews and then pull into whichever maint makes sense.

[akleshchev]

Done in inventory_favorites branch, waiting for other favorite related changes before merging into a proper branch

[dmytrohproductengine]

Passed QA.

Verified on the Second Life Release 7.1.12.12258017789 (64bit) on Win10/OSX in the scope of IQA#414.

Also, the favorite item disappears from the "Favorites" tab after adding and then removing its parent folder from the "Favorites" tab (see secondlife/viewer/issues/3353).