Allow ignoring snapped items via Alt key (fixes #28022)

[bakajikara]

Resolves: #28022

By holding down the Alt key while editing, you can now edit the target line segment without affecting snapped items.

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[mike-spa]

I would suggest passing EditData& to this function, and creating the variable

bool moveSnapped = !(ed.modifiers & AltModifier);

here. That way, if the behaviour needs to be changed in future it can be done by changing a single piece of code instead of two.