Add a home screen widget drawer

[Djspaceg]

The home screen should have more widgets available than just the 5 that are present now. (In fact, the home screen should only have 4 primary widget slots on it, to avoid clutter. Because of this, we should have a place to store and access the additional widgets that are available for use on the home screen. A drawer that stores the unused widgets would be useful for this purpose. The drawer likely should not render the widget in place, for speed reasons, but when dragged into a slot should render immediately once dropped onto a slot (swapping with the widget it's replacing). The macOS "Dashboard" concept would be a good model to look to for inspiration for how adding widgets could work.

• This drawer will need to have some activator toggle, perhaps the edit-layout-arrangement button/state could be the toggle?
• We may want the drawer to animate so factor that possibility into the design.
• Each theme may desire to customize the position/orientation/appearance of the drawer, so that aspect of its design will be important.

The drawer will hopefully become a first-class Agate component, based on expected usage scenarios, so we'll want it to be extensible and reusable in other scenarios beyond this one.

Please comment or ask questions in person if you have any.