Add noUnusedLocals tsconfig flag, fix issues

Two parts were commented out:
- _writeStopped: This is because _xoffSentToCatchUp references this variable
  and the feature is sort of half implemented/disabled.
- _clearChar: This is because some comments still reference this, it's part
  of the work needed to do better line redrawing.

src/SelectionManager.ts

@@ -3,11 +3,10 @@
  * @license MIT
  */
 
-import { ITerminal, ICircularList, ISelectionManager, IBuffer, LineData, CharData, XtermListener } from './Types';
+import { ITerminal, ISelectionManager, IBuffer, CharData, XtermListener } from './Types';
 import { MouseHelper } from './utils/MouseHelper';
 import * as Browser from './shared/utils/Browser';
 import { CharMeasure } from './utils/CharMeasure';
-import { CircularList } from './utils/CircularList';
 import { EventEmitter } from './EventEmitter';
 import { SelectionModel } from './SelectionModel';
 import { CHAR_DATA_WIDTH_INDEX, CHAR_DATA_CHAR_INDEX } from './Buffer';