Fix behavior of More... links by adding a Link class

We had problems because the More links were reacting on press but
other elements were reacting on release. (Often the link would trigger
*and* an item.) Just connecting to ::button-release-event on
ClutterText gives a stuck grab (since ClutterText gets the press
but not the release), so we need more complicated code that we
encapsulate into a new class.

link.js: new "pseudo-widget" that implements a clickable link.
overlay.js: Use Link.Link for the More.. links

http://bugzilla.gnome.org/show_bug.cgi?id=573323

js/ui/link.js

@@ -0,0 +1,81 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+const Clutter = imports.gi.Clutter;
+const Lang = imports.lang;
+const Signals = imports.signals;
+
+// Link is a clickable link. Right now it just handles properly capturing
+// press and release events and short-circuiting the button handling in
+// ClutterText, but more features like different colors for hover/pressed states
+// or a different mouse cursor could be implemented.
+//
+// The properties passed in are forwarded to the Clutter.Text() constructor,
+// so can include, 'text', 'font_name', etc.
+function Link(props) {
+    this._init(props);
+}
+
+Link.prototype = {
+    _init : function(props) {
+	let realProps = { reactive: true };
+        // The user can pass in reactive: false to override the above and get
+        // a non-reactive link (a link to the current page, perhaps)
+	Lang.copyProperties(props, realProps);
+
+	this.actor = new Clutter.Text(realProps);
+	this.actor._delegate = this;
+	this.actor.connect('button-press-event', Lang.bind(this, this._onButtonPress));
+	this.actor.connect('button-release-event', Lang.bind(this, this._onButtonRelease));
+	this.actor.connect('enter-event', Lang.bind(this, this._onEnter));
+	this.actor.connect('leave-event', Lang.bind(this, this._onLeave));
+
+	this._buttonDown = false;
+	this._havePointer = false;
+    },
+
+    // Update the text of the link
+    setText : function(text) {
+	this.actor.text = text;
+    },
+
+    // We want to react on buttonDown, but if we override button-release-event for
+    // ClutterText, but not button-press-event, we get a stuck grab. Tracking
+    // buttonDown and doing the grab isn't really necessary, but doing it makes
+    // the behavior perfectly correct if the user clicks on one actor, drags
+    // to another and releases - that should not trigger either actor.
+    _onButtonPress : function(actor, event) {
+	this._buttonDown = true;
+	this._havePointer = true; // Hack to work around poor enter/leave tracking in Clutter
+	Clutter.grab_pointer(actor);
+
+	return true;
+    },
+
+    _onButtonRelease : function(actor, event) {
+	if (this._buttonDown) {
+	    this._buttonDown = false;
+	    Clutter.ungrab_pointer(actor);
+
+	    if (this._havePointer)
+		this.emit('clicked');
+	}
+
+	return true;
+    },
+
+    _onEnter : function(actor, event) {
+	if (event.get_source() == actor)
+	    this._havePointer = true;
+
+        return false;
+    },
+
+    _onLeave : function(actor, event) {
+	if (event.get_source() == actor)
+	    this._havePointer = false;
+
+        return false;
+    }
+};
+
+Signals.addSignalMethods(Link.prototype);