dash: Remove show/hide functions

As Main.overview is now usable from the dash's constructor, move
the setup of signal connections there and remove the show/hide
methods which were used as workaround.

https://bugzilla.gnome.org/show_bug.cgi?id=642196

js/ui/overview.js

@@ -537,7 +537,6 @@ Overview.prototype = {
 
         this.viewSelector.show();
         this._workspacesDisplay.show();
-        this.dash.show();
 
         this.workspaces = this._workspacesDisplay.workspacesView;
         global.overlay_group.add_actor(this.workspaces.actor);
@@ -751,7 +750,6 @@ Overview.prototype = {
 
         this._workspacesDisplay.hide();
         this.viewSelector.hide();
-        this.dash.hide();
 
         this._desktopFade.hide();
         this._background.hide();