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
- Create SingleInstance class to handling single instance stuff.
- Instead of calling getInstance() everywhere, initialize each object
explicitly in order when application starts and getInstance()s do not
create instances any more and they merely return the created
instances.
- Only set initial values in construtor functions __init__(). Move the
initialization of context-aware (i.e. things that depend on other
things) to separate functions.
- Split application creation and initialziation into several steps and
them should be called explicitly in the correct order.
