qt: merge ME edition into SE

(breaks PE temporarily)

Adds a Standard/Music Application Mode button to SE and thus adds the
ability to run ME scan types in SE. When in Music mode, the
Music-specific results window, details panel and preferences panel will
show up.

All preferences except scan_type become shared between app modes
(changing the pref in a mode changes it in the other mode).

Results Window and Details Panel are now re-created at each scan
operation because they could change their type between two runs.

Preferences panel is instantiated on the fly and discarded after close.

This is a very big merge operation and I'm trying to touch as little
code as possible, sometimes at the cost of elegance. I try to minimize
the breakage that this change brings.

core/tests/app_test.py

@@ -384,7 +384,7 @@ class TestCaseDupeGuruWithResults:
         app.JOB = Job(1, lambda *args, **kw: False) # Cancels the task
         add_fake_files_to_directories(app.directories, self.objects) # We want the scan to at least start
         app.start_scanning() # will be cancelled immediately
-        eq_(len(self.rtable), 0)
+        eq_(len(app.result_table), 0)
 
     def test_selected_dupes_after_removal(self, do_setup):
         # Purge the app's `selected_dupes` attribute when removing dupes, or else it might cause a