Add cover art option in export settings

[bmatherly]

As suggested here:
https://forum.shotcut.org/t/how-to-set-ehhmm-cover-art/48515

Depends on mltframework/mlt#1207

[Copilot]

Pull request overview

Adds a “Cover Art” picker to the Encode dock UI and wires it into preset load/save so an image path can be persisted via the attached_pic consumer property.

Changes:

• Add Cover Art label + path field + browse button to the Encode “Other” tab UI.
• Load attached_pic into the new field when a preset is selected, and emit it back into collected properties during export.
• Implement the browse button slot to select an image file and update the stored open path.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/docks/encodedock.ui	Adds Cover Art UI controls (label, line edit, browse button) to the “Other” tab.
src/docks/encodedock.h	Declares the new auto-connected slot for the Cover Art browse button.
src/docks/encodedock.cpp	Clears/loads attached_pic, sets it during property collection, and implements the file picker slot.

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[bmatherly]

Here is a screenshot of the advanced export "Other" tab with the cover art selection added

[ddennedy]

Hide the new UI element if MLT version is not sufficient at build time. Require at least 7.37.0 not future version 7.38.0. (If someone is building against an old git master of the never released version 7.37 that is their problem.)

Also, consider to add a new tab named "Metadata" that would contain this as well as other fields:
https://forum.shotcut.org/t/embedding-metadata-into-exports/14806

In the future it will have an option to embed chapter marks (similar behavior for selection as File > Export > Markers as Chapters)

[bmatherly]

Hide the new UI element if MLT version is not sufficient at build time. Require at least 7.37.0

Done

Also, consider to add a new tab named "Metadata" that would contain this as well as other fields

Done. I also implemented the fields documented in the Documentation.

I also put the "Disable Subtitles" option in that tab.

Discussion:

• What do you think about the "Cover Art" label? Should we all it something else?
• I wonder if we should invert the "Disable Subtitles" option so that it is "Enable Subtitles" and checked by default.

[ddennedy]

* What do you think about the "Cover Art" label? 

That's fine

* I wonder if we should invert the "Disable Subtitles" option so that it is "Enable Subtitles" and checked by default.

Disable is consistent with Disable video and Disable audio

[Copilot]

Pull request overview

Adds a new Metadata tab to Shotcut’s Export/Encode dock to let users set common container metadata and (when supported by libmlt) pick an image to embed as cover art.

Changes:

• Adds a new “Metadata” tab to the export UI, including a cover art picker and fields for common tags (title/artist/comment/copyright/date/description/genre/language).
• Loads/saves these values through MLT preset properties (meta.attr.*.markup, attached_pic, and alang) during preset load and export property collection.
• Adds basic input validation for date (ISO-8601 YYYY-MM-DD) and language (3-letter code), and hides cover art controls on older libmlt versions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/docks/encodedock.ui	Adds the Metadata tab UI (cover art + metadata fields) and updates tab order / moves “Disable subtitles” into Metadata.
src/docks/encodedock.h	Declares the new on_coverArtButton_clicked() slot.
src/docks/encodedock.cpp	Implements metadata load/save, validators, libmlt-version UI gating, and the cover art file picker dialog.

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[bmatherly]

I think this is OK to merge.

A couple of notes:
In the VLC Media Info tab, the date is only displayed as a year. I don't really know if this is normal. But it appears as a standard ISO date in FFProbe.

In the VLC Media Info tab, the copyright is not shown for MP4 files (it is shown for MP3 and MKV). I was able to add some custom fields using com.apple.date and some MOV info tags to make it appear. But the tricks make the code really ugly, and I do not think it is worth it.