DashboardLayout with additional API

[alalonde]

We have a requirement for a Widget that has much of the functionality of a DockPanel but without the concept of tabs and a more granular API. This is my initial stab at such functionality. A DashboardLayout is essentially a DockPanel with the following modifications:

1. Panel areas do not have a built-in TabBar. It is up to the Widget to define its own "header" and/or drag and drop functionality if desired.
2. You cannot drop a widget "on top of" another widget since there is no concept of stacked areas within a panel. All other insertion types provided by the DockPanel are supported.
3. There are a few additional signals to capture more granular events from a DashboardPanel: areaResized which is called when a handle is dragged, widgetRemoved, and layoutRearranged which is fired when a widget is dragged and dropped from one place within the panel to another.
4. A resizeLayoutArea method allows explicitly setting the relative sizes of a split area. In order to accomplish this, areas within a DashboardPanel are assigned UUIDs at creation.

We will likely need some of this functionality in a DockPanel as well, but this can serve as at least a starting point for discussion of some of the Signals and mutation API methods that would provide much more granular programmatic interactions. There is a great deal of duplicated code between this and the DockPanel which of course is not ideal. I'm up for suggestions on refactoring / extracting common functionality elsewhere.

I included an example implementation in the example-dashboardpanel package demonstrating much of the functionality, including save/restore and drag and drop.