Massive unused/unnecessary variable sweep-up, and more

[ferdnyc]

As the branch name indicates, this started as just a set of fixes for one specific issue: the throwaway half of the tuple returned by QFileDialog.getOpenFileName(). But, it ballooned into a lot more. All of the changes are centered around the theme of reducing unused-and-unnecessary variables (both to quiet warnings from static analysis, and to streamline the code).

Fix QFileDialog.getOpenFileName() calls

• Ignore file_type return value, when not used (which is always)
  Apparently the typical pattern for this is:

  filename, _ = QFileDialog.getOpenFileName(...)

  But we can't use that, because we've bound _ to the translation wrapper. So, I just went with:

  filename = QFileDialog.getOpenFileName(...)[0]

• simplify some logic around empty return values (canceled dialogs)
• Create fewer variables we don't need/use

Similar cleanup with the use of os.path standard library functions

• I went through EVERY .py file in the repo, and looked at all of the uses of the os.path library functions, looking to reduce the number of one-sided os.path.split() / os.path.splitext() calls and the unused variables they engendered. Meaning...

  # If x or y is unused in:
  (x, y) = os.path.split(f)
  # then replace it with whichever is appropriate of:
  x = os.path.dirname(f)
  y = os.path.basename(f)
  
  # If m or n is unused in:
  (m, n) = os.path.splitext(f)
  # then replace it with whichever is appropriate of:
  m = os.path.splitext(f)[0]
  n = os.path.splitext(f)[1]

• Also, cut down on indentation by reworking some loops like:
  Old...

  for x in y:
      if A:
          ...
          if B:
              do_real_work()

  New...

  for x in y:
      if not A:
          continue
      ...
      if not B:
          continue
      do_real_work()

• Rewrap some really wide / heavily indented lines

Clean up app vs get_app() usage in main_window.py

• In functions that call get_app() frequently, make sure we've set app = get_app() at the top, and replace every remaining instance of get_app() with app.
• In functions where we don't access app/get_app() often, eliminate app and just use get_app() directly.

[jonoomph]

Looking good! conflicts though

[ferdnyc]

Resolved, but the final_cut_pro.py exporter conflict was huge, like70% of the entire file. I just blindly took the PR branch version, I'll check now whether that accidentally lost any changes from develop.

[ferdnyc]

Looks fine, and Travis is happy. Crossing my fingers and merging.