Refactor import file: is_same_file function

[nickleus27]

#2403
Made new function in canvasview.cpp called is_same_file() This puts the code from import_file() that checks if file selected is same file into a separate bool function to make import_file() cleaner. I am unsure if this new helper function should be a private or public function. At the moment I have it as public.

[ice0]

Merged. Thank you!