gtk: accept dropping URLs from browsers onto the main window (#2232)

Unlike files, which come as a URI list, links dragged from browsers have
textual selection types:

- Firefox: text/plain;charset=utf-8
- Chrome: UTF8_STRING

If the URI list for files is empty, try to pass the selection text to
add_from_url().

gtk/Application.cc

@@ -815,11 +815,23 @@ void Application::Impl::on_drag_data_received(
 {
     auto const uris = selection_data.get_uris();
 
-    auto files = std::vector<Glib::RefPtr<Gio::File>>();
-    files.reserve(uris.size());
-    std::transform(uris.begin(), uris.end(), std::back_inserter(files), &Gio::File::create_for_uri);
+    if (!uris.empty())
+    {
+        auto files = std::vector<Glib::RefPtr<Gio::File>>();
+        files.reserve(uris.size());
+        std::transform(uris.begin(), uris.end(), std::back_inserter(files), &Gio::File::create_for_uri);
+
+        open_files(files);
+    }
+    else
+    {
+        auto const text = gtr_str_strip(selection_data.get_text());
 
-    open_files(files);
+        if (!text.empty())
+        {
+            core_->add_from_url(text);
+        }
+    }
 
     drag_context->drag_finish(true, false, time_);
 }
@@ -840,6 +852,7 @@ void Application::Impl::main_window_setup()
     /* register to handle URIs that get dragged onto our main window */
     wind_->drag_dest_set(Gtk::DEST_DEFAULT_ALL, Gdk::ACTION_COPY);
     wind_->drag_dest_add_uri_targets();
+    wind_->drag_dest_add_text_targets(); /* links dragged from browsers are text */
     wind_->signal_drag_data_received().connect(sigc::mem_fun(*this, &Impl::on_drag_data_received));
 }