Cross-virtual dom signals

## Specific Demand

Signals currently use the context API to communicate with eachother. This works great within one virtual dom, but once you create a new virtual dom it can cause issues. For example, you cannot create a signal in one window and send it to another window in this [example](https://github.com/DioxusLabs/dioxus/blob/master/examples/multiwindow.rs)

## Implement Suggestion

We could switch from the context API to a global lazy static/oncecell runtime