Menu-like behavior

[RobertOrthofer]

This PR implements changes according to #15

• The default size has been set via the prop of the v-autocomplete, the wrapping element has been removed, as well as the now-obsolete css import of the example application. Now it should integrate like a normal v-autocomplete, although some styling props like density, single-line or min-width are set by the plugin itself.
• The menu-like behavior is now implemented with a proper v-menu, that has the v-autocomplete as the target, but no activator, so we still have full control. Vuetify detaches menus from the DOM, and uses <teleport> to "attach" them back to the location of their target, which makes them appear over any other element, even when used in a navigation bar with overflow: hidden, like in the example application.
• some good-practives have been implemented for the v-list-item, but there is still a well-known bug in vuetify that throws warnings for nodes with duplicate IDs inside a v-virtual-scoll element.

[ahocevar]

Great work! I only added a minor suggestion you might want to address.