Allow new project creation to create in a folder that contains files but NOT a RasCAL-2 project

[alexhroom]

Depends on #3 and #21

Currently (in PR #18) we have made it so that a RasCAL-2 project can only be initialised in an empty folder. However, once the contents of a RasCAL-2 project are well-defined (#21) then we can detect the presence of a RasCAL-2 project in a folder and allow a new project to be created in non-empty folders without projects.

Acceptance:

• Create a list of RasCAL-2 project files and a routine to detect them
• Change the startup widget to allow a RasCAL-2 project to be saved to a non-empty folder which does not contain any project files.