Rubberband fix for selecting large area in Songeditor

[BaraMGB]

fixes #4974

scrolling and zooming while selecting an area works now. But with 180 additions and 70 deletions this is a big bug fix. I don't know if this should be merged so close before releasing 1.2.

[PhysSong]

I don't know if this should be merged so close before releasing 1.2.

If you want to test more, we can include this in 1.2.1 instead of 1.2.0.

[PhysSong]

This is the first part of my review. I will review this PR again in a few days.

[BaraMGB]

Thank you for your time.

[BaraMGB]

@Reflexe Thanks a lot for reviewing this.
@PhysSong Do you have any objection about this PR?

[PhysSong]

Here are minor comments regarding whitespaces. I'm testing the functionality and will let you know if there are any issues or not.

[PhysSong]

I found that lassoing with Shift+drag on the timeline is broken. @BaraMGB Could you look into it?

[BaraMGB]

@PhysSong Thank you. I'll look into it. 👍

[BaraMGB]

@PhysSong can this be merged or do you want to test it more?

[PhysSong]

The functionality looks okay, but I think we have some time to test this more because this PR targets the stable branch.

[BaraMGB]

@PhysSong Do you think, this should go into 1.2.1?

[PhysSong]

I think it's okay to merge this now. As far as I tested, this won't bring any serious regressions.