use stat_mtime for gen hash (as moving a file does not change content) AND for fall-back date/time of importing a photo -- Fixes BUG-72

BUGs

@@ -6,5 +6,3 @@ BUG-61: in Fullscreen mode and next/prev dropped out of FS when calling /viewlis
         -- then we can stay on-page, and stay in FS and then just call ViewImageOrVide()
 BUG-69: when moving a directory (maybe files???) it creates new destination directory even if one exists... not sure what the os.replace, etc. does at that point???
 BUG-70: when a dir is a match and a file is a match (common with our date-based matching), then we get 2 answers in the search
-BUG-72: when re-scanning import dir, I always regen hash for 20201225_181843.jpg - why?
-        -- also happened for a lot of storage photos, see job: 909

pa_job_manager.py

@@ -1128,7 +1128,7 @@ def GetDateFromFile(file, stat):
             check2 = datetime( year, month, day )
         # give up and use file sys date
         except:
-            year, month, day, _, _, _, _, _, _ = datetime.fromtimestamp(stat.st_ctime).timetuple()
+            year, month, day, _, _, _, _, _, _ = datetime.fromtimestamp(stat.st_mtime).timetuple()
     c=date(year, month, day).isocalendar()
     woy=c[1]
     return year, month, day, woy
@@ -1349,7 +1349,7 @@ def GenHashAndThumb(job, e):
     if job.current_file_num % 100 == 0:
         session.commit()
     stat = os.stat( e.FullPathOnFS() )
-    if stat.st_ctime < e.file_details.last_hash_date:
+    if stat.st_mtime < e.file_details.last_hash_date:
         if DEBUG:
             print(f"OPTIM: GenHashAndThumb {e.name} file is older than last hash, skip this")
         job.current_file_num+=1