resources: Use standard icon path

GtkApplication by default appends "icons" to the app id and add that
to the icon search path.  So let's just use that instead of adding
custom one.

This commit also fixes missing icon for "Add call" button in active
call window

src/calls-application.c

@@ -303,7 +303,6 @@ static void
 startup (GApplication *application)
 {
   g_autoptr(GtkCssProvider) provider = NULL;
-  GtkIconTheme *icon_theme;
   g_autoptr(GError) error = NULL;
 
   G_APPLICATION_CLASS (calls_application_parent_class)->startup (application);
@@ -318,9 +317,6 @@ startup (GApplication *application)
   g_set_prgname (APP_ID);
   g_set_application_name (_("Calls"));
 
-  icon_theme = gtk_icon_theme_get_default ();
-  gtk_icon_theme_add_resource_path (icon_theme, "/sm/puri/calls/");
-
   g_action_map_add_action_entries (G_ACTION_MAP (application),
                                    actions,
                                    G_N_ELEMENTS (actions),

src/calls.gresources.xml

@@ -15,6 +15,8 @@
   </gresource>
   <gresource prefix="/sm/puri/calls/">
     <file>style.css</file>
+  </gresource>
+  <gresource prefix="/sm/puri/Calls/icons/">
     <file>new-call-symbolic.svg</file>
     <file>call-arrow-incoming-symbolic.svg</file>
     <file>call-arrow-incoming-missed-symbolic.svg</file>

src/ui/call-display.ui

@@ -184,7 +184,7 @@
                                   <object class="GtkImage">
                                     <property name="visible">True</property>
                                     <property name="can_focus">False</property>
-                                    <property name="resource">/sm/puri/calls/new-call-symbolic.svg</property>
+                                    <property name="icon_name">new-call-symbolic</property>
                                     <property name="icon_size">3</property>
                                     <property name="vexpand">True</property>
                                   </object>