gtkgui: Add image preview to file chooser

Moved the file chooser dlg of "Add images" into own class.

Added an image preview widget to dialog.

Code is based on
https://github.com/endlessm/eos-photos/blob/bc787c9f43a99576fcdfca15505dd8438424aeef/src/widgets/preview_file_chooser_dialog.py

Fixes: #39

photocollage/gtkgui.py

@@ -26,7 +26,7 @@
 import cairo
 import gi
 gi.require_version('Gtk', '3.0')  # noqa
-from gi.repository import Gtk, Gdk, GObject
+from gi.repository import Gtk, Gdk, GObject, GdkPixbuf
 from six.moves import urllib  # Python 2 backward compatibility
 
 from photocollage import APP_NAME, artwork, collage, render
@@ -282,14 +282,11 @@ def update_photolist(self, new_images):
             dialog.destroy()
 
     def choose_images(self, button):
-        dialog = Gtk.FileChooserDialog(_("Choose images"),
-                                       button.get_toplevel(),
-                                       Gtk.FileChooserAction.OPEN,
-                                       select_multiple=True)
-        dialog.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.CANCEL)
-        dialog.add_button(Gtk.STOCK_OK, Gtk.ResponseType.OK)
-
-        set_open_image_filters(dialog)
+        dialog = PreviewFileChooserDialog(title=_("Choose images"),
+                                          parent=button.get_toplevel(),
+                                          action=Gtk.FileChooserAction.OPEN,
+                                          select_multiple=True,
+                                          modal=True)
 
         if dialog.run() == Gtk.ResponseType.OK:
             files = dialog.get_filenames()
@@ -769,6 +766,44 @@ def __init__(self, parent, message):
         self.show_all()
 
 
+class PreviewFileChooserDialog(Gtk.FileChooserDialog):
+    PREVIEW_MAX_SIZE = 256
+
+    def __init__(self, **kw):
+        super(PreviewFileChooserDialog, self).__init__(**kw)
+
+        self.add_button(Gtk.STOCK_CANCEL, Gtk.ResponseType.CANCEL)
+        self.add_button(Gtk.STOCK_OK, Gtk.ResponseType.OK)
+
+        set_open_image_filters(self)
+
+        self._preview = Gtk.Image()
+        # Don't let preview size down horizontally for skinny images, cause
+        # that looks distracting
+        self._preview.set_size_request(
+            PreviewFileChooserDialog.PREVIEW_MAX_SIZE, -1)
+        self.set_preview_widget(self._preview)
+        self.set_use_preview_label(False)
+        self.connect("update-preview", self.update_preview_cb)
+
+    def update_preview_cb(self, file_chooser):
+        filename = self.get_preview_filename()
+        if filename is None or os.path.isdir(filename):
+            self.set_preview_widget_active(False)
+            return
+        try:
+            pixbuf = GdkPixbuf.Pixbuf.new_from_file_at_size(
+                filename,
+                PreviewFileChooserDialog.PREVIEW_MAX_SIZE,
+                PreviewFileChooserDialog.PREVIEW_MAX_SIZE)
+            self._preview.set_from_pixbuf(pixbuf)
+        except Exception as e:
+            print(e)
+            self.set_preview_widget_active(False)
+            return
+        self.set_preview_widget_active(True)
+
+
 def main():
     # Enable threading. Without that, threads hang!
     GObject.threads_init()