[IMP] tags: introduce xml tag

Big change! This commit introduces an xml function tag to easily define
inline templates.

This is a pretty big change toward single file owl components

Part of #284

src/qweb/extensions.ts

@@ -227,7 +227,9 @@ QWeb.addDirective({
   atNodeEncounter({ ctx, value }): boolean {
     const slotKey = ctx.generateID();
     ctx.rootContext.shouldDefineOwner = true;
-    ctx.addLine(`const slot${slotKey} = this.slots[context.__owl__.slotId + '_' + '${value}'];`);
+    ctx.addLine(
+      `const slot${slotKey} = this.constructor.slots[context.__owl__.slotId + '_' + '${value}'];`
+    );
     ctx.addIf(`slot${slotKey}`);
     ctx.addLine(
       `slot${slotKey}.call(this, context.__owl__.parent, Object.assign({}, extra, {parentNode: c${ctx.parentNode}, vars: extra.vars, parent: owner}));`

src/qweb/qweb.ts

@@ -143,15 +143,16 @@ export class QWeb extends EventBus {
 
   static TEMPLATES: { [name: string]: Template } = {};
 
+  static nextId: number = 1;
+
   h = h;
   // dev mode enables better error messages or more costly validations
   static dev: boolean = false;
 
-
   // slots contains sub templates defined with t-set inside t-component nodes, and
   // are meant to be used by the t-slot directive.
-  slots = {};
-  nextSlotId = 1;
+  static slots = {};
+  static nextSlotId = 1;
 
   // recursiveTemplates contains sub templates called with t-call, but which
   // ends up in recursive situations.  This is very similar to the slot situation,