frontend: use searchable dropdowns

Current behaviour

Various Cockpit pop-up windows use a select dropdown with a separate text entry to filter the options, which is visually unpleasant and unintuitive. This pattern is currently used where one or more data-lake variable needs to be selected (e.g. the plotter widget config / the custom action triggers config), as well as the MAVLink message inspector.

Expected or desired behaviour

Vuetify includes a v-autocomplete element, to combine this functionality into a searchable select element, while still supporting multi-selection and the like.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.