[IMP] owl: add basic support for sub roots

In this commit, we extend the owl App class to support multiple sub
roots. This is useful for situations where we want to mount sub
components in non-managed DOM. This is exactly what the Knowledge app is
doing, with mounting views in an html editor.

Currently, this requires some difficult and fragile hacks, and still,
the result is that it is very easy to mix components from the main App
and a SubApp.  But Knowledge does not actually care about creating a sub
app. It only needs the possibility to mount sub components in dynamic
places.

closes #1640

tests/components/__snapshots__/props_validation.test.ts.snap

@@ -924,6 +924,20 @@ exports[`props validation props: list of strings 1`] = `
 }"
 `;
 
+exports[`props validation validate props for root component 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><block-text-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['message'];
+    return block1([txt1]);
+  }
+}"
+`;
+
 exports[`props validation validate simple types 1`] = `
 "function anonymous(app, bdom, helpers
 ) {