ivtcDupeRemover: discarding configuration dialog adds the filter nevertheless

[eumagga0x2a]

STR:

• Load a video in Avidemux, press Ctrl+Alt+F to open the video filter manager.
• Select "Interlacing" category, right click on "Remove IVTC dupe." --> "Add".
• Discard the configuration dialog.

Actual Results:
The filter gets added nevertheless.

Expected Results:
Discarding the dialog cancels the operation, the filter is not added to the list of active filters.

[eumagga0x2a]

Fixed by [ivtcDupeRemover] Actually use the dialog return value, thanks!