Prevent accidental drags by adding drag distance threshold (3.x)

[Calinou]

3.x version of #49310. Thanks @foxydevloper for the initial implementation!

Note that this PR uses a lowered threshold as requested (8 pixels instead of 10 pixels), and the 2D editor threshold is now scaled with the zoom to behave consistently at all zoom levels.

[xix-xeaon]

I just want to say, the kind of work you do is often thankless, since each thing is just a small thing. But you do many of these small things and they add up. You are solving issues that thousands of users are dealing with every day but often aren't worth reporting on - many don't even realize they're issues.

Each one individually of no great importance but together they become the entire user experience. The work you do ensures that Godot attains a level of professionalism seen in commercial projects and elevates it far above the sum of it's parts. Thank you!

I'm singling out @Calinou here because I'm constantly seeing his name over and over in the pull requests, but anyone else who submits small, seemingly unworthy, fixes (even just a single one) know that you too deserve to bask in a share of this glory =)

[akien-mga]

Thanks!