Several 'History' fixes.

History is now loaded explicitly rather than on first access.
History will now purge non-existent files from its record as it is loaded.
The 'last open project' is only opened automatically on startup if that project was actually open when the pipelinetool was closed.
History was incorrectly recording project files that had not yet been saved to disk (eg, import / newproject ).
I thought that Reset seemed to imply the clear operation was also going to be serialized to disk, which it does not do, while Clear does not have that implication in my mind.