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