[WIP][Vega] Implement context filter modification

elastic#17210

Testing code (click button)

```
{
  "$schema": "https://vega.github.io/schema/vega/v3.json",
  "marks": [
    {
      "name": "myButton",
      "type": "rect",
      "encode": {
        "enter": {
          "xc": {"signal": "width/2"},
          "yc": {"signal": "height/2"},
          "width": {"signal": "width*0.8"},
          "height": {"signal": "height*0.8"},

          "cornerRadius": {"value": 6},
          "strokeWidth": {"value": 10}
        },
        "update": {
          "stroke": {"value": "gray"},
          "fill": {"value": "lightgray"}
        },
        "hover": {"fill": {"value": "gray"}}
      }
    }
  ],
  "signals": [
    {
      "name": "%ADD_FILTER%",
      "on": [
        {
          "events": "@mybutton:click",
          "update": "{field: 'SRC', value: 10, operator: 'IS'}"
        }
      ]
    }
  ]
}
```

package.json

@@ -207,6 +207,7 @@
     "validate-npm-package-name": "2.2.2",
     "vega-lib": "^3.2.1",
     "vega-lite": "^2.3.1",
+    "vega-spec-injector": "^0.0.2",
     "vega-schema-url-parser": "1.0.0",
     "vision": "4.1.0",
     "webpack": "3.6.0",

src/core_plugins/vega/public/data_model/vega_parser.js

@@ -8,6 +8,7 @@ import hjson from 'hjson';
 import { Utils } from './utils';
 import { EmsFileParser } from './ems_file_parser';
 import { UrlParser } from './url_parser';
+import Vsi from 'vega-spec-injector';
 
 const DEFAULT_SCHEMA = 'https://vega.github.io/schema/vega/v3.0.json';
 
@@ -80,6 +81,10 @@ export class VegaParser {
     }
 
     this._calcSizing();
+
+    const vsi = new Vsi(this._onWarning.bind(this));
+
+    vsi.addToList(this.spec, `signals`, ['%ADD_FILTER%']);
   }
 
   /**

src/core_plugins/vega/public/vega_view/vega_base_view.js

@@ -143,6 +143,29 @@ export class VegaBaseView {
     return false;
   }
 
+  setView(view) {
+    this._view = view;
+    if (view) {
+      view.addSignalListener('%ADD_FILTER%',
+        /**
+         * @param {string} sig signal name
+         * @param {object} value filter object to add
+         * @param {string} value.field name of the filter field
+         * @param {string|number} value.value the value of the filter
+         * @param {string} value.operator how value should be compared. "IS" by default.
+         */
+        (/* sig, value */) => {
+
+
+
+          // TODO: add new filter
+
+
+        }
+      );
+    }
+  }
+
   /**
    * Set global debug variable to simplify vega debugging in console. Show info message first time
    */

src/core_plugins/vega/public/vega_view/vega_view.js

@@ -17,11 +17,11 @@ export class VegaView extends VegaBaseView {
     if (this._parser.useHover) view.hover();
 
     this._addDestroyHandler(() => {
-      this._view = null;
+      this.setView(null);
       view.finalize();
     });
 
     await view.runAsync();
-    this._view = view;
+    this.setView(view);
   }
 }