Fix empty ignore List dialog bug in PE

Re-instantiating a new scanner for PE  made the ignore list dialog
target the wrong ignore list. We now only instantiate a scanner once.

Fixes #253

core/app.py

@@ -151,9 +151,9 @@ class DupeGuru(Broadcaster):
     # show_problem_dialog()
     # select_dest_folder(prompt: str) --> str
     # select_dest_file(prompt: str, ext: str) --> str
-    
-    # in fairware prompts, we don't mention the edition, it's too long.
+
     PROMPT_NAME = "dupeGuru"
+    SCANNER_CLASS = scanner.Scanner
     
     def __init__(self, view):
         if view.get_default(DEBUG_MODE_PREFERENCE):
@@ -166,7 +166,7 @@ class DupeGuru(Broadcaster):
             os.makedirs(self.appdata)
         self.directories = directories.Directories()
         self.results = results.Results(self)
-        self.scanner = scanner.Scanner()
+        self.scanner = self.SCANNER_CLASS()
         self.options = {
             'escape_filter_regexp': True,
             'clean_empty_dirs': False,

core_pe/app.py

@@ -17,10 +17,10 @@ from .result_table import ResultTable
 class DupeGuru(DupeGuruBase):
     NAME = __appname__
     METADATA_TO_READ = ['size', 'mtime', 'dimensions', 'exif_timestamp']
+    SCANNER_CLASS = ScannerPE
     
     def __init__(self, view):
         DupeGuruBase.__init__(self, view)
-        self.scanner = ScannerPE()
         self.scanner.cache_path = op.join(self.appdata, 'cached_pictures.db')
     
     def _get_dupe_sort_key(self, dupe, get_group, key, delta):