Add drag screen

[mogimogitomato]

I wanted to move the screen when the mouse is held down and dragged,
so I made it possible to perform screen dragging by detect mousedown + mousemove.

PS:

Thank you for the nice gem!
I was looking for a way to output column comments on ER diagrams, and I found this gem.
Thanks for the multifunctionality.

[kjcpaas]

I've started using this gem recently loving it so far!

Would love to see this feature added in as it is very useful.

[k-nasa]

I'm finding this Gem very useful. Thank you! I think this feature is extremely helpful, and I'm looking forward to future additions.

[shifakhan]

This looks like a useful feature, hope it gets merged soon!

[unchidev]

@mogimogitomato

Thank you for your PR!
This functionality has been incorporated in PR #138. I'll close this PR for the following reasons 🙏🏿:

1. The proposed implementation prevents text selection via drag interaction
2. For mouse-based operations, we wanted to provide it together with zoom functionality

Thank you for contributing to improve the project. Looking forward to your future enhancements!

[mogimogitomato]

@unchidev

I tried v0.6.0, so cool !!! and very useful, I agree with the thoughts you commented on.
Also my code was imperfect (Ex: The use of movement x/y should have been prudent... and so on),
the PR #138 diff was very informative for me.

Thank you for contributing to improve the project. Looking forward to your future enhancements!

Thank you as well! If I come up with something else, I will be happy to make suggestions!