Use GtkFileChooserNative

Use GtkFileChooserNative [1] instead of GtkFileChooserDialog [2]
to integrate better with the platform (e.g. use the portal
implementation when run with GTK_USE_PORTAL=1, resulting in
the KDE Frameworks implementation being used when
xdg-desktop-portal-kde is in use). Quoting from
the GtkFileChooserDialog doc [2]:

> If you want to integrate well with the platform you should use the
> GtkFileChooserNative API, which will use a platform-specific dialog if
> available and fall back to GtkFileChooserDialog otherwise.

Also replace the use of GTK_STOCK_CANCEL [3] and GTK_STOCK_OPEN [4]
which were deprecated in GTK 3.10:

Both, the `accept_label` and `cancel_label` params of
`Gtk.FileChooserNative.new` can be `None` to use the default
text ("Open", "Cancel"). [5]

Adjust the only caller (in `vmmVMWindow#_takeScreenshot`)
that was passing an explicit label/icon name for the
accept button to pass `_("_Save")` as label, rather than
the also deprecated Gtk.STOCK_SAVE [6].
(GtkFileChooserDialog has special handling for Gtk.STOCK_SAVE
etc., but that's not generally the case for native dialogs).

Rename the method param from `choose_button` to `choose_label`
to make clearer that this is a label.

[1] https://docs.gtk.org/gtk3/class.FileChooserNative.html
[2] https://docs.gtk.org/gtk3/class.FileChooserDialog.html
[3] https://docs.gtk.org/gtk3/const.STOCK_CANCEL.html
[4] https://docs.gtk.org/gtk3/const.STOCK_OPEN.html
[5] http://pygobject-doc.gitee.io/pgi-docs/Gtk-3.0/classes/FileChooserNative.html#Gtk.FileChooserNative.new
[6] https://docs.gtk.org/gtk3/const.STOCK_SAVE.html

Fixes #315

virtManager/error.py

@@ -234,7 +234,7 @@ class vmmErrorDialog(vmmGObject):
 
     def browse_local(self, dialog_name, start_folder=None,
                      _type=None, dialog_type=None,
-                     choose_button=None, default_name=None,
+                     choose_label=None, default_name=None,
                      confirm_overwrite=False):
         """
         Helper function for launching a filechooser
@@ -246,19 +246,11 @@ class vmmErrorDialog(vmmGObject):
         """
         if dialog_type is None:
             dialog_type = Gtk.FileChooserAction.OPEN
-        if choose_button is None:
-            choose_button = Gtk.STOCK_OPEN
 
-        buttons = (Gtk.STOCK_CANCEL,
-                   Gtk.ResponseType.CANCEL,
-                   choose_button,
-                   Gtk.ResponseType.ACCEPT)
-
-        fcdialog = Gtk.FileChooserDialog(title=dialog_name,
+        fcdialog = Gtk.FileChooserNative.new(title=dialog_name,
                                              parent=self.get_parent(),
                                              action=dialog_type,
-                                    buttons=buttons)
-        fcdialog.set_default_response(Gtk.ResponseType.ACCEPT)
+                                             accept_label=choose_label)
 
         if default_name:
             fcdialog.set_current_name(default_name)

virtManager/vmwindow.py

@@ -558,7 +558,7 @@ class vmmVMWindow(vmmGObjectUI):
             _("Save Virtual Machine Screenshot"),
             _type=("png", _("PNG files")),
             dialog_type=Gtk.FileChooserAction.SAVE,
-            choose_button=Gtk.STOCK_SAVE,
+            choose_label=_("_Save"),
             start_folder=start_folder,
             default_name=default,
             confirm_overwrite=True)