Add ability to load projects and convert R1 projects

[alexhroom]

Depends on #3, #21, would be easier after/alongside #22

May not be fully possible until RascalSoftware/python-RAT#76 is resolved.

Currently (in PR #18) the startup widget buttons to load an existing project or convert a RasCAL-1 project do not work. This functionality should be added.

I would suggest that the RasCAL-1 project is converted to a RasCAL-2 project in the same folder, in order to preserve paths to custom files etc.

Acceptance:

• Create a routine to detect that a project exists in a folder (also required for Allow new project creation to create in a folder that contains files but NOT a RasCAL-2 project #22) and load the project/controls (and any other important data) from that project into the model
• Create a routine which takes a RasCAL-1 .mat file, converts it using RATapi.utils.convert.r1_to_project_class(), and loads the resulting project into the model (with default controls?)

Other desirable functionality:

• maybe a 'Recent Projects' tab on the 'load existing project' dialog so that it is easy to get back to where you were