Added Auto-Expand Folder On Unread option

Summary:
This patch adds a new option, to auto-expand folders whenever any of its
children have unread content. This makes it quicker to navigate to new
content when using folders.

akregator_part.rc:
    - added an action to <Menu name="view"> with name=auto_expand_folders
    - bumped version

settings_general.ui:
    - added kcfg_AutoExpandFolders field under General->Global

subscriptionlistview:
    - added slotSetAutoExpandFolders to expand any unread folders when
      enabling the auto_expand_folders setting

actionmanagerimpl:
    - added code to slotSettingsChanged, to update the auto_expand_folders action state
    - added a KAction to ActionManagerImpl:initSubscriptionListView,
      connected to SubscriptionListView::slotSetAutoExpandFolders

selectioncontroller:
    - added subscriptionDataChanged to update folder expand state for
      newly updated items
    - connect m_subscriptionModel's dataChanged signal to
      subscriptionDataChanged

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: dvratil, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8165