filter section and dropdown

This PR adds two new components.

t-filters-dropdown: Shows filter with their respective sections assigned using filter-type prop, clicking on any filter adds the filter to filters state.
t-filters-section: Main container that shows all the selected filters.

Syntax

<t-filters-section>
  <t-select v-show="filters.hasOwnProperty('urlparam1')" label="Type" filter-type="ISSUES" t-layer="layer_1" t-filter:selected_items="urlparam1"  />
  <t-select v-show="filters.hasOwnProperty('urlparam2')" label="Type" filter-type="PROJECTS" t-layer="layer_2" t-filter:selected_items="urlparam2"  />
</t-filters-section>

Important

filter-type helps in categorising of filters in their sections.
v-show along with hasOwnProperty is used on filters object instead of v-if because when a filter is selected, it will have an empty string value which will have undesirable effects with v-if, also because our filters only get data when clicked or opened v-show will not affect backend performance.

topcoat-data / topcoat-public

filter section and dropdown #162