feat: add manual movement start toggle on systems

[galshacham]

Adds a right-click menu action on systems to manually mark/unmark a system as a movement start, independent of automatic indentation detection. This is useful for exercise books and anthologies where multiple pieces share a page without visual indentation.

Changes:

• New SystemIndentTask for undo/redo support
• New toggleIndentation method in InterController
• New rebuildPages method in SystemManager
• "Mark/Unmark as movement start" action in System context menu

After

Being able to separate scores manually and not only with indentations

Tested locally

• toggled several systems as new movment and rebuilt scores -> allowed extra exports
• toggled several systems as not movments and rebuilt scores -> returned to original state

This code was written mainly by claude,
I wanted this for something and thought it might come handy as a good feature

[hbitteur]

@galshacham

Thank you for this PR and congratulations on your understanding of the Audiveris code! (even though Claude was involved…)

My only concern is that you created this PR on the "master" branch, whereas we only develop on the "development" branch (or any temporary sub-branch of "development").

Indeed, we try to use the "master" branch only for releases (and we merge "development" into "master" specifically for releases).

Could you redirect your PR to the Audiveris "development" branch?
(Sorry, I'm not very familiar with this procedure, so I can't tell you if it's easy to do.)

Otherwise, I'll first try merging your PR into one of my local "development" branches so I can test everything locally.

Please keep me updated.

[hbitteur]

@galshacham
I'm currently testing your PR locally.
I noticed that the Score menu item is not correctly updated in the popup menu, after a system has been marked as a new movement start
I'm debugging
Stay tuned

[galshacham]

My only concern is that you created this PR on the "master" branch, whereas we only develop on the "development" branch (or any temporary sub-branch of "development").

Done, mb! thanks for notifing.

I noticed that the Score menu item is not correctly updated in the popup menu, after a system has been marked as a new movement start

As far as I understood the workflow of Audiveris, its part of the routine of "recreating scroes" in "Book" tab.

Indeed after Rebuilding scores, the score numbers are updated at the right click toggle bar.

However, I believe its possible to auto update it once clicking, though it might affect performance a bit.