(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/CustomSectionElement.ts

@@ -0,0 +1,47 @@
+import { SafeBrowser, ViewProcess } from 'app/client/lib/SafeBrowser';
+import { PluginInstance } from 'app/common/PluginInstance';
+
+export { ViewProcess } from 'app/client/lib/SafeBrowser';
+
+/**
+ * A PluginCustomSection identifies one custom section in a plugin.
+ */
+export interface PluginCustomSection {
+  pluginId: string;
+  sectionId: string;
+}
+
+export class CustomSectionElement {
+
+  /**
+   * Get the list of all available custom sections in all plugins' contributions.
+   */
+  public static getSections(plugins: PluginInstance[]): PluginCustomSection[] {
+    return plugins.reduce<PluginCustomSection[]>((acc, plugin) => {
+      const customSections = plugin.definition.manifest.contributions.customSections;
+      const pluginId = plugin.definition.id;
+      if (customSections) {
+        // collect identifiers
+        const sectionIds = customSections.map(section => ({sectionId: section.name, pluginId}));
+        // concat to the accumulator
+        return acc.concat(sectionIds);
+      }
+      return acc;
+    }, []);
+  }
+
+  /**
+   * Find a section matching sectionName in the plugin instances' constributions and returns
+   * it. Returns `undefined` if not found.
+   */
+  public static find(plugin: PluginInstance, sectionName: string): ViewProcess|undefined {
+    const customSections = plugin.definition.manifest.contributions.customSections;
+    if (customSections) {
+      const section = customSections.find(({ name }) => name === sectionName);
+      if (section) {
+        const safeBrowser = plugin.safeBrowser as SafeBrowser;
+        return safeBrowser.createViewProcess(section.path);
+      }
+    }
+  }
+}