Add KSharedConfig::openStateConfig for storing state information

Summary:
A common complaint is that our apps store 'state' information such as recent files or window sizes in their configuration files, making it ugly to store them in version control systems.

Therefore we should not store such information in XDG_CONFIG_DIR but instead use XDG_DATA_DIR for this.

This patch adds a utility method that creates a KSharedConfig backed by a file in such a suitable location.

For e.g. dolphin the file .local/share/dolphin/dolphinstaterc would be created.

See T12246 for some context

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26440