Added menu control and generalized vertical flow container.

Summary: This implements a main menu / menu item control for use in the asset manager.  It also generalizes the `VerticalContainer` as `FlowContainer` to allow for both vertical and horizontal flows.

Test Plan: Run the asset manager (either standalone or with the game).  The menu control should behave, within reason, like a normal menu control.

Reviewers: Sabre070

Maniphest Tasks: T51

Differential Revision: http://code.redpointsoftware.com.au/D56