Draft: Commit save and restore session without xfce session manager

Xfce apps are widely used in non-Xfce environments because of their relatively small amount of dependencies and solid feature set.

This MR is compatible with the existing restore session functionality provided by xfce-session-manager and aims to provide session restoration for non-Xfce environments.

Additionally a new --restore command-line option is provided which synergizes with --window and --tab.

Issue #75