(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/lib/ObservableSet.js

@@ -0,0 +1,74 @@
+var _ = require('underscore');
+var ko = require('knockout');
+var dispose = require('./dispose');
+
+/**
+ * An ObservableSet keeps track of a set of values whose membership is controlled by a boolean
+ * observable.
+ * @property {ko.observable<Number>} count: Count of items that are currently included.
+ */
+function ObservableSet() {
+  this._items = {};
+  this.count = ko.observable(0);
+}
+dispose.makeDisposable(ObservableSet);
+
+/**
+ * Adds an item to keep track of. The value is added to the set whenever isIncluded observable is
+ * true. To stop keeping track of this item, call dispose() on the returned object.
+ *
+ * @param {ko.observable<Boolean>} isIncluded: observable for whether to include the value.
+ * @param {Object} value: Arbitrary value. May be omitted if you only care about the count.
+ * @return {Object} Object with dispose() method, which can be called to unsubscribe from
+ *    isIncluded, and remove the value from the set.
+ */
+ObservableSet.prototype.add = function(isIncluded, value) {
+  var uniqueKey = _.uniqueId();
+  var sub = this.autoDispose(isIncluded.subscribe(function(include) {
+    if (include) {
+      this._add(uniqueKey, value);
+    } else {
+      this._remove(uniqueKey);
+    }
+  }, this));
+
+  if (isIncluded.peek()) {
+    this._add(uniqueKey, value);
+  }
+
+  return {
+    dispose: function() {
+      this._remove(uniqueKey);
+      this.disposeDiscard(sub);
+    }.bind(this)
+  };
+};
+
+/**
+ * Returns an array of all the values that are currently included in the set.
+ */
+ObservableSet.prototype.all = function() {
+  return _.values(this._items);
+};
+
+/**
+ * Internal helper to add a value to the set.
+ */
+ObservableSet.prototype._add = function(key, value) {
+  if (!this._items.hasOwnProperty(key)) {
+    this._items[key] = value;
+    this.count(this.count() + 1);
+  }
+};
+
+/**
+ * Internal helper to remove a value from the set.
+ */
+ObservableSet.prototype._remove = function(key) {
+  if (this._items.hasOwnProperty(key)) {
+    delete this._items[key];
+    this.count(this.count() - 1);
+  }
+};
+
+module.exports = ObservableSet;