layout: Construct the primary monitors's hot corner, too

This cleans up the code considerably, and makes it so that
one path creates all hot corners for all monitors. Why this
wasn't done originally, I have no clue...

The one complication is debouncing if the button and hot corner
are triggered in rapid succession, so we just move this tracking
to the overview.

https://bugzilla.gnome.org/show_bug.cgi?id=663661

js/ui/overview.js

@@ -33,6 +33,8 @@ const SHADE_ANIMATION_TIME = .20;
 
 const DND_WINDOW_SWITCH_TIMEOUT = 1250;
 
+const OVERVIEW_ACTIVATION_TIMEOUT = 0.5;
+
 const ShellInfo = new Lang.Class({
     Name: 'ShellInfo',
 
@@ -146,6 +148,8 @@ const Overview = new Lang.Class({
         this._backgroundGroup.hide();
         this._bgManagers = [];
 
+        this._activationTime = 0;
+
         this.visible = false;           // animating to overview, in overview, animating out
         this._shown = false;            // show() and not hide()
         this._shownTemporarily = false; // showTemporarily() and not hideTemporarily()
@@ -536,6 +540,7 @@ const Overview = new Lang.Class({
         this.visible = true;
         this.animationInProgress = true;
         this.visibleTarget = true;
+        this._activationTime = Date.now() / 1000;
 
         // All the the actors in the window group are completely obscured,
         // hiding the group holding them while the Overview is displayed greatly
@@ -633,6 +638,20 @@ const Overview = new Lang.Class({
             this.show();
     },
 
+    // Checks if the Activities button is currently sensitive to
+    // clicks. The first call to this function within the
+    // OVERVIEW_ACTIVATION_TIMEOUT time of the hot corner being
+    // triggered will return false. This avoids opening and closing
+    // the overview if the user both triggered the hot corner and
+    // clicked the Activities button.
+    shouldToggleByCornerOrButton: function() {
+        if (this.animationInProgress)
+            return false;
+        if (this._activationTime == 0 || Date.now() / 1000 - this._activationTime > OVERVIEW_ACTIVATION_TIMEOUT)
+            return true;
+        return false;
+    },
+
     //// Private methods ////
 
     _syncInputMode: function() {