Add option to pass CDK Portals instead of templates

[DaSchTour]

Currently customization of components is done through templates added as content children. This may lead to a lot of duplicate code in templates. As primeng already uses @angular/cdk and has it as a dependency it may be useful to allow passing portals into components. These could easily be reused.
For example icons could be easily created through a factory including an already existing icon component and massing the icon as data to the portal.

So for example considering the pick list code could look like this

<p-picklist [templates]="templates" …></picklist>

templates = {
  moveupicon: this.createIcon('chevron-up')
  movetopicon: this.createIcon('chevrons-up')
  movedownicon: this.createIcon('chevron-down')
  movebottomicon: this.createIcon('chevrons-down')
  movetotargeticon: this.createIcon('chevron-right')
  movealltotargeticon:  this.createIcon('chevrons-right')
  movetosourceicon: this.createIcon('chevron-left')
  movealltosourceicon: this.createIcon('chevrons-left')
  targetfiltericon: this.createIcon('magnifying-glass')
  sourcefiltericon: this.createIcon('magnifying-glass')
}

createIcon(name: string) {
  const injector = Injector.create({
    providers: [{provide: ICON_NAME, useValue: name}]
  });
  return new ComponentPortal({component: IconComponent, injector })
}