feature(Dropdown): component initialization

[Stradivario]

Issue

Description

Introduces rx-dropdown component

Screenshots (optional)

Usage

import { DropdownComponent } from '@rxdi/ui-kit/dropdown';

@Module({
  components: [DropdownComponent]
})
export class AppModule {}

Inside html

<rx-dropdown
  .menuItems=${[
    html`
      <a href="#" class="item">You need to setup your SSL certificate!</a>
    `,
    html`
      <a href="#" class="item">You need to setup your SSL certificate!</a>
    `,
    html`
      <a href="#" class="item">You need to setup your SSL certificate!</a>
    `
  ]}
>
  <div style="margin-right: 10px;margin-top: 3px;">
    ${notification}
  </div>
</rx-dropdown>