Registering custom events

## Specific Demand

An easy way to add events to Dioxus without the need to mess with the source code.

## Implement Suggestion

Something that would allow a usage like, for example:
```rust
register_event! {
// attribute name
onmyevent;
// setup structure
MyeventHandler;
// elements on which it can be set
div
p
header
}

struct MyeventHandler {
...
}

impl MyeventHandler {
// Setup the event (create needed structures, hooks...)
// should be called when dioxus starts
fn new() -> Self {
...
}

// Connect a callback
fn connect(element, callback) -> Result<...> {
...
}

// Disconnect a callback
fn disconnect(element) -> Result<...> {
...
}
}

fn App(cx: Scope) -> Element {
render! {
div {
onmyevent: |event| {
...
}
}
}
```