feature: wfs filtering (origo-map#1680)

Fixes origo-map#1679
Adds setFilter and clearFilter methods to the wfs source
Example:
origo.api().getLayer('my_layer').getSource().setFilter("namn in ('Museiparken')")

src/layer/wfssource.js

@@ -63,6 +63,23 @@ class WfsSource extends VectorSource {
       .catch(() => failure());
   }
 
+  /**
+   * Set filter on layer
+   * @param {any} cql
+   */
+  setFilter(cql) {
+    this._options.filter = cql;
+    this.refresh();
+  }
+
+  /**
+   * Clear filter on layer
+   */
+  clearFilter() {
+    this._options.filter = '';
+    this.refresh();
+  }
+
   /**
    * Helper to reuse code. Consider it to be private to this class
    * @param {any} extent