Fixes flake8 error F401.
Most errors were fixed by running
autoflake -i -r --remove-all-unused-imports .
which also removes unnecessary 'pass' statements, some by hand-editing.
Create the dialog buttons with GtkDialog.add_buttons(). This places them
either to the dialog action area, or to the headerbar, accoording to the
system setting.
Also set 'do-overwrite-confirmation' property of the Dialog to False.
gpodderexporttolocalfolder.ui:
* Save with Glade 3.38.2
* Replace GTK stock labels with text
exportlocal.py: Set transient for parent (was not set before)
with a checkbox to export all remaining episodes to same folder.
This restores the ability to choose the filename for each episode,
while keeping export of a large number of episodes convenient (click export remaining...)
Teemu Ikonen
Ana Hosu
Eric Le Lay