[IMP] slots: via prop 'slots'

The slot inner working has been reworked. A prop "slots" is now passed
explicitely to the component. It looks like

{ slotName_1: slotInfo_1, ..., slotName_m: slotInfo_m }

with the objects slotInfo_i with mandatory keys "__render", "__ctx",
and optional key "__scope" and possibly others.

Here is how a slotInfo object can be created:
A slotInfo object is normally created by setting in a template something
like

<div>
    <t t-set-slot="foo" t-set-scope="scope" param_1="var" param_2="3">
        content
        <t t-esc="scope.bool"/>
        <t t-esc="scope.num"/>
    </t>
</div>

and it will be used somewhere like

<div>
    <t t-esc="props.slots.foo.param_1"/>
    <t t-slot="foo" bool="other_var" num="5">
</div>

In the above example, the function "__render" produces the block dom
element for the content of the t-set-slot.
The context "__ctx" will have a key "scope" with value { bool: ..., num: 5 }
and "__scope" will be set to "scope".

src/app/template_helpers.ts

@@ -17,12 +17,17 @@ function callSlot(
   parent: any,
   key: string,
   name: string,
-  defaultSlot?: (ctx: any, key: string) => BDom,
-  dynamic?: boolean
+  dynamic: boolean,
+  extra: any,
+  defaultSlot?: (ctx: any, key: string) => BDom
 ): BDom {
-  const slots = ctx.__owl__.slots;
-  const slotFn = slots[name];
-  const slotBDom = slotFn ? slotFn(parent, key) : null;
+  const slots = (ctx.props && ctx.props.slots) || {};
+  const { __render, __ctx, __scope } = slots[name] || {};
+  const slotScope = Object.create(__ctx || {});
+  if (__scope) {
+    slotScope[__scope] = extra || {};
+  }
+  const slotBDom = __render ? __render(slotScope, parent, key) : null;
   if (defaultSlot) {
     let child1: BDom | undefined = undefined;
     let child2: BDom | undefined = undefined;