Always create an enclosing directory for new projects

As part of the project settings changes in 6.0, an enclosing directory will be required for projects (i.e. it will no longer be permitted to have more than one project file in a directory). This is the first step in that direction: get rid of the checkbox on the new project dialog, and replace it with a note that a new directory will be created.

We also now reject an attempt to load a .pro file from a directory that contains more than one.