Refactor char atlas to enable generation with worker

This doesn't actually add workers but it separates the code nicely
such that the minimal amount of relevant code can be pulled in by
worker code.

Part of #955

src/SelectionManager.ts

@@ -4,7 +4,7 @@
  */
 
 import { MouseHelper } from './utils/MouseHelper';
-import * as Browser from './utils/Browser';
+import * as Browser from './shared/utils/Browser';
 import { CharMeasure } from './utils/CharMeasure';
 import { CircularList } from './utils/CircularList';
 import { EventEmitter } from './EventEmitter';