Fixes #1478 - Append new sLog when reloading in set command.

* Migrate weak reference into logger class so we can test clearing it.
* Ensure new state.log is propoagted into settings on `set` command.
* Fix set test so that it ensures the sLog is relatively stable when
  reloading on set command.
Add PR notes.
Merge pull request #1486 from sbt/fix/issue-1478

Fixes #1478 - Append new sLog when reloading in set command.
Fixes #1306 - Attempt to turn test suite names into better filenames.
Add notes for test report fix.
Merge pull request #1487 from sbt/wip/fr-1306

Fixes #1306 - Attempt to turn test suite names into better filenames.
Fixes #1375 - Check for empty filenames.

Starting sbt in root isn't a great idea, but it shouldn't break.
Add notes so we have a merge conflict.

Conflicts:
	notes/0.13.6.md
Merge pull request #1488 from sbt/wip/fix-1375

Fixes #1375 - Check for empty filenames.
Fixes #856 - Store ++ scala version changes on session.

* Ensure the ++ command stores its changes on the sbt session.
* Make sure `session clear-all` will clear out ++ changes
* Validate that `set` command doesn't undo `++` changes

Note: There is some autogenerated Setting[_] delegate optimisation
      work that could be done in the future.
Add notes for bugfix.