Add Playhead and Timeline to Pattern Editor

[regulus79]

This PR adds a visual indicator to show the pattern editor playing, along with a timeline widget to allow the user to skip forward/backward in a pattern.

Addresses #1195

2025-03-18.17-00-04.mp4

I noticed that the timeline arrow position moves smoothly while the step highlighting moves in steps, which kind of looks weird. Are you guys okay with that, or do you have any ideas for a better solution?

Changes

• Added a TimeLineWidget to PatternEditor, and set it up in the constructor, along with some helper variables (the track head width, which was copied from the Song Editor, and the max number of steps in the clips to calculate the pixels per bar, which has its own function to update)
• Added a new icon, step_btn_highlight, to overlay the steps which are currently being played.
• Added a getter function, TrackView::getClipViews() to allow the Pattern Editor to access the midi clip views to update them when the playhead moves. Previously, afaik, once the clip views are created, they leave the scope and cannot be accessed again, which is a problem.
• I was not able to neatly solve the issue of reordering the clip views when the clips get reordered (when pattern tracks are moved around in the song editor), so currently I loop over all of them to update them. It's not great, but it works.
• Added a getter function MidiClip::steps() to allow the pattern editor to know how long the clips are, and update the pixelsPerBar for the timeline accordingly (since we don't have scrolling yet; maybe this will change in the future). On second, this could probably be achieved via MidiClip::length(), but it probably doesn't hurt to handle it with steps. I can rework this if anyone wants.

Note: the current signal/slot system for the pattern editor is very odd; updatePosition doesn't seem to be called when the playback position changes, which means that I had to make it be triggered by something else. I have decided to connect it to TimeLineWidget::positionChanged, which works, but this may have to be reworked when #7454 is completed.

[regulus79]

Edit: Fixed. I decided to add a new signal to PatternStore which is triggered by the midi clips when the time signature changes, which in turn is connected to update the pattern editor timeline stuff.

Okay--I'm having a bit of an issue updating the timeline length when the time signature changes, since the midi clips respond to the Song::timeSignatureChanged signal after the pattern editor, so I can't get the updated clip lengths. I could potentially recompute them right there, or I could retrigger MidiClip::changeTimeSignature(), or perhaps something else. I may have to think about this.

[regulus79]

I have decided to make the code simpler by using the clip length instead of the clip steps. This means I do not need to edit MidiClip.h

[regulus79]

Do you guys want me to add some kind of timeline playhead for normal midi clips/sample clips/automation clips in the pattern editor?