Try: Extensions API

[timmyc]

Related to: #3, and much of the description copied from there.

Design Background: p6riRB-35x-p2

Based off some of the early design mocks, discussions around them, and our primary goal of the Dashboard being built with Extensibility First - one of the key features of the project will be the ability for internal core devs, and extension developers to build custom blocks, and for users to add them to the dashboard.

While the designs are still being finalized, we can still use an early mock-up to see how "slots" to house custom blocks could potentially be utilized:

Goals
For this Try Issue, the goal is to further explore the Extensions API concept ( p90Yrv-Ae-p2 ) that @justinshreve presented at our meetup.

Ideally to vet this idea it would be nice to see the basic woo-dash page have:

• Two slot-fill target regions ( maybe Main and sidebar )
• Example blocks that are registered via PHP, and subsequently rendered into slots

Discussion
While these items are out of scope of the try branch I wanted to capture some other questions I have been pondering around this topic:

• Do we need to support priority of insertion into fill areas?
• There has been chatter around customization of the dashboard, could users re-arrange blocks in fill areas to customize?

[justinshreve]

Since we've finished our exploration for now, is it OK to close these two issues?