Enhance the process of adding subtitles to the ListView by refactoring the code and introducing asynchronous file processing. This includes consolidating redundant methods into a single method that generates the VideoPreviewGeneratorSub object and ensuring null values are handled. Improved user experience by setting the cursor to a wait state during background operations.

Signed-off-by: Ivandro Jao <Ivandrofly@gmail.com>

Display a "please wait" message and change the cursor to a wait cursor when files are being processed in the drag-and-drop operation. Clear the message and reset the cursor once processing is complete.

Signed-off-by: Ivandro Jao <Ivandrofly@gmail.com>

Implemented a visible progress bar to give user feedback during drag-and-drop operations in the GenerateVideoWithSoftSubs form. This enhances the UI by indicating processing status and improving user experience.

Signed-off-by: Ivandro Jao <Ivandrofly@gmail.com>

This commit deletes the unused variable _inputVideoFileName from the GenerateVideoWithSoftSubs class. This cleanup improves code readability and reduces potential confusion during future maintenance.

Signed-off-by: Ivandro Jao <Ivandrofly@gmail.com>

Wrap subtitle list operations within BeginUpdate and EndUpdate calls to reduce UI flicker and improve performance during batch item insertions.

Signed-off-by: Ivandro Jao <Ivandrofly@gmail.com>

Extracted wait state activation and deactivation into separate methods: `ActivateWaitState` and `DeactivateWaitState`. This improves code readability and reduces repetition.

Signed-off-by: Ivandro Jao <Ivandrofly@gmail.com>