When importing only the models of a project file, Cura wasn't checking whether the file should
be added to the recent files list. This commit fixes that.
CURA-7996
The logic using just a "skip_project_file_check" boolean was too obscure
imho, so used a string value with more explicit values
to improve readability
CURA-6824
The UM.Application.platform property has been removed by someone for quite some time.
I don't know why the check was in there, but it doesn't seem needed at all as the property was always undefined