src: Change the way we handle static stylesheet

Previously we deduped duplicate styles from being added to the
document.head by only adding the own method of a component.

This time we keep a reference to the static function in a
seen array and only call it once.

We expose a new static method called `registerStyles()` which
can be called by any child class that wants to inherit and
overwrite styles

For example

```js
class MyDialog extends Dialog {
  static stylesheet () {
    Tonic.registerStyles(super.stylesheet)
    return 'CSS'
  }
}
```

index.js

@@ -95,15 +95,18 @@ class Tonic extends window.HTMLElement {
     Tonic._tags = Object.keys(Tonic._reg).join()
     window.customElements.define(htmlName, c)
 
-    Tonic.addStyles(c)
+    if (c.stylesheet) {
+      Tonic.registerStyles(c.stylesheet)
+    }
   }
 
-  static addStyles (c) {
-    if (Object.prototype.hasOwnProperty.call(c, 'stylesheet')) {
-      const styleNode = document.createElement('style')
-      styleNode.appendChild(document.createTextNode(c.stylesheet()))
-      if (document.head) document.head.appendChild(styleNode)
-    }
+  static registerStyles (stylesheetFn) {
+    if (Tonic._seenStylesheets.includes(stylesheetFn)) return
+    Tonic._seenStylesheets.push(stylesheetFn)
+
+    const styleNode = document.createElement('style')
+    styleNode.appendChild(document.createTextNode(stylesheetFn()))
+    if (document.head) document.head.appendChild(styleNode)
   }
 
   static escape (s) {
@@ -351,6 +354,7 @@ Object.assign(Tonic, {
   _states: {},
   _children: {},
   _reg: {},
+  _seenStylesheets: [],
   _index: 0,
   version: typeof require !== 'undefined' ? require('./package').version : null,
   SPREAD: /\.\.\.\s?(__\w+__\w+__)/g,