appDisplay: Reimplement well layout to be width-independent

Use ShellGenericContainer to implement a fully dynamic layout
for the application well.  It's still fixed to 4 columns by default,
but no longer requires a fixed width to be passed in on start.

With another chunk of work, it could likely try to adjust to
the case where we can only fit fewer than 4 items in the well.

Remove the border highlighting on mouseover, since that caused
reallocations, and the grid layout isn't trivial.

Delete the unused shell_global_get_word_with function.

js/ui/dash.js

@@ -428,7 +428,7 @@ Dash.prototype = {
 
         this._appsContent = new Big.Box({ orientation: Big.BoxOrientation.HORIZONTAL });
         this._appsSection.append(this._appsContent, Big.BoxPackFlags.EXPAND);
-        this._appWell = new AppDisplay.AppWell(this._width);
+        this._appWell = new AppDisplay.AppWell();
         this._appsContent.append(this._appWell.actor, Big.BoxPackFlags.EXPAND);
 
         this._moreAppsPane = null;