(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/sessionObs.ts

@@ -0,0 +1,53 @@
+/**
+ * createSessionObs() creates an observable tied to window.sessionStorage, i.e. preserved for the
+ * lifetime of a browser tab for the current origin.
+ */
+import {safeJsonParse} from 'app/common/gutil';
+import {IDisposableOwner, Observable} from 'grainjs';
+
+/**
+ * Creates and returns an Observable tied to sessionStorage, to make its value stick across
+ * reloads and navigation, but differ across browser tabs. E.g. whether a side pane is open.
+ *
+ * The `key` isn't visible to the user, so pick any unique string name. You may include the
+ * docId into the key, to remember a separate value for each doc.
+ *
+ * To use it, you must specify a default, and a validation function: this module exposes a few
+ * helpful ones. Some examples:
+ *
+ *    panelWidth = createSessionObs(owner, "panelWidth", 240, isNumber);  // Has type Observable<number>
+ *
+ *    import {StringUnion} from 'app/common/StringUnion';
+ *    const SomeTab = StringUnion("foo", "bar", "baz");
+ *    tab = createSessionObs(owner, "tab", "baz", SomeTab.guard);  // Type Observable<"foo"|"bar"|"baz">
+ */
+export function createSessionObs<T>(
+  owner: IDisposableOwner|null,
+  key: string,
+  _default: T,
+  isValid: (val: any) => val is T,
+) {
+  function fromString(value: string|null): T {
+    const parsed = value == null ? null : safeJsonParse(value, null);
+    return isValid(parsed) ? parsed : _default;
+  }
+  function toString(value: T): string|null {
+    return value === _default || !isValid(value) ? null : JSON.stringify(value);
+  }
+
+  const obs = Observable.create<T>(owner, fromString(window.sessionStorage.getItem(key)));
+  obs.addListener((value: T) => {
+    const stored = toString(value);
+    if (stored == null) {
+      window.sessionStorage.removeItem(key);
+    } else {
+      window.sessionStorage.setItem(key, stored);
+    }
+  });
+  return obs;
+}
+
+/** Helper functions to check simple types, useful for the `isValid` argument to createSessionObs. */
+export function isNumber(t: any): t is number { return typeof t === 'number'; }
+export function isBoolean(t: any): t is boolean { return typeof t === 'boolean'; }
+export function isString(t: any): t is string { return typeof t === 'string'; }