[gnome-documents] properties: avoid using instanceof

[Cosimo Cecchi <cosimoc src gnome org>]

commit c28f8bcbc603aeb7f0e9f7b8b1a1a4f3a7b8020b
Author: Cosimo Cecchi <cosimoc gnome org>
Date:   Fri Aug 2 18:29:03 2019 +0200

    properties: avoid using instanceof
    
    Instead add a method to the document class to know whether its title
    can be edited, false by default.

 src/documents.js  | 8 ++++++++
 src/properties.js | 3 +--
 2 files changed, 9 insertions(+), 2 deletions(-)
---
diff --git a/src/documents.js b/src/documents.js
index a8e385c1..5c81d77a 100644
--- a/src/documents.js
+++ b/src/documents.js
@@ -464,6 +464,10 @@ const DocCommon = new Lang.Class({
         throw(new Error('DocCommon implementations must override canEdit'));
     },
 
+    canEditTitle: function() {
+        return false;
+    },
+
     canShare: function() {
         throw(new Error('DocCommon implementations must override canShare'));
     },
@@ -920,6 +924,10 @@ var LocalDocument = new Lang.Class({
         return this.collection;
     },
 
+    canEditTitle: function() {
+        return true;
+    },
+
     canShare: function() {
         return false;
     },
diff --git a/src/properties.js b/src/properties.js
index 3647ef35..88a52f18 100644
--- a/src/properties.js
+++ b/src/properties.js
@@ -27,7 +27,6 @@ const _ = imports.gettext.gettext;
 const C_ = imports.gettext.pgettext;
 
 const Application = imports.application;
-const Documents = imports.documents;
 const Mainloop = imports.mainloop;
 const TrackerUtils = imports.trackerUtils;
 
@@ -122,7 +121,7 @@ var PropertiesDialog = new Lang.Class({
         grid.add (this._docType);
 
         // Title value
-        if (doc instanceof Documents.LocalDocument) {
+        if (doc.canEditTitle()) {
             this._titleEntry = new Gtk.Entry({ activates_default: true,
                                                text: doc.name,
                                                editable: true,