sessions: upgrade anonymous session to regular one

This is a attempt to make the "default" session more easier to handle for users. Reported bug https://bugs.kde.org/show_bug.cgi?id=268913

General idea:

• anonymous session is
  • still without name (and not explicitly displayed)
  • is saved whenever the named session would be
  • is saved to "Last session" so it can be restored (if kate sessions are configured to restore the last one on start)
• its possible to store the configuration (plugins loaded, etc...) as session default
• the new session is created from defaults and stored as anonymous

Scenarios:

1. if user doesn't actively work with sessions, it will still get the session manager dialog at the start
2. if user configured "restore last session", but doesn't work with sessions otherwise, the kate will use anonymous session under the hood
3. if user triggers "new session" and current one is anonymous then the kate will ask user what to do with the old one (save it under name or just overwrite it with the new one)
4. setting defaults for new session is now explicitly exposed to user by "Save as default" menu item.

TODOs:

• actual filesystem session storage is at this point a little bit of mess
• the session manager would deserve some interface cleanup
• starting new session from command line with something stored in anonymous session, will get the anonymous session wiped out
• it's not possible to create a session with factory defaults easily