add "reading" filter for books currently being read

front/src/BooksBrowser.vue

@@ -9,6 +9,7 @@ import Pagination from './Pagination.vue'
 const FilterStates = Object.freeze({
     READ: "read",
     WANTREAD: "wantread",
+    READING: "reading",
 });
 
 const limit = 6;
@@ -58,6 +59,11 @@ function pageChange(newPageNumber) {
             :class="computeDynamicClass(FilterStates.READ)">
       {{$t('bookbrowser.read')}}
     </button>
+    <button class="button is-medium"
+            @click="onFilterButtonClick(FilterStates.READING)"
+            :class="computeDynamicClass(FilterStates.READING)">
+      {{$t('bookbrowser.reading')}}
+    </button>
     <button class="button is-medium"
             @click="onFilterButtonClick(FilterStates.WANTREAD)"
             :class="computeDynamicClass(FilterStates.WANTREAD)">