Add new search addon, move AddonManager to public

Because addons build on top of the API it needs to live in public, the main
reason for this is because the implementation of buffer differs on
public/Terminal and src/Terminal.

src/public/AddonManager.ts

@@ -0,0 +1,56 @@
+/**
+ * Copyright (c) 2019 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { ITerminalAddon, IDisposable, Terminal } from 'xterm';
+
+export interface ILoadedAddon {
+  instance: ITerminalAddon;
+  dispose: () => void;
+  isDisposed: boolean;
+}
+
+export class AddonManager implements IDisposable {
+  protected _addons: ILoadedAddon[] = [];
+
+  constructor() {
+  }
+
+  public dispose(): void {
+    for (let i = this._addons.length - 1; i >= 0; i--) {
+      this._addons[i].instance.dispose();
+    }
+  }
+
+  public loadAddon(terminal: Terminal, instance: ITerminalAddon): void {
+    const loadedAddon: ILoadedAddon = {
+      instance,
+      dispose: instance.dispose,
+      isDisposed: false
+    };
+    this._addons.push(loadedAddon);
+    instance.dispose = () => this._wrappedAddonDispose(loadedAddon);
+    instance.activate(<any>terminal);
+  }
+
+  private _wrappedAddonDispose(loadedAddon: ILoadedAddon): void {
+    if (loadedAddon.isDisposed) {
+      // Do nothing if already disposed
+      return;
+    }
+    let index = -1;
+    for (let i = 0; i < this._addons.length; i++) {
+      if (this._addons[i] === loadedAddon) {
+        index = i;
+        break;
+      }
+    }
+    if (index === -1) {
+      throw new Error('Could not dispose an addon that has not been loaded');
+    }
+    loadedAddon.dispose();
+    loadedAddon.isDisposed = true;
+    this._addons.splice(index, 1);
+  }
+}