Add modifer key for zoom steps in 2d editor

[TheDuriel]

Godot version:

3.1

Zooming on a large map 16x16k pixels, a mandatory task for quick traversal from one end to the other, is very slow. Maybe we can add a speed toggle via hotkey (shift or similar), or at least an option in the scene editor to change the speed.

[Calinou]

Fixed by #48252 and #50490 (use Alt + Mouse wheel).