Fix and improvements to saving and new tabs

newFile now just calls newTab but with 'alsoSave' param, which is picked
up and runs saveFile, giving more power to the API
When opening a file, if it's a new tab, call createNewTab with a 'new'
param, which is picked up and if set, we don't call setPreviousFiles. We
don't want to set them on adding a new tab anyway, plus it creates an
extra process, which means the server queue gets jammed and new tabs
don't get a save action processed.