This feature is mainly designed for developers who uses touchpad instead of mouse, and want to scroll instead of zoom. Every macOS developers will like it since it feels intuitive.
2 new settings are added to `2d_editor`:
1. `scroll_to_pan`: turn on to use mouse/touchpad scroll to pan canvas item editor view instead of zoom
2. `pan_speed`: use this value to change scroll speed
Enabled by default as in Blender, but can be disabled separately for 2D & 3D;
the core functionality is in Input so this could be reused or even exposed to scripts in the future
The other subfolders of tools/ had already been moved to either
editor/, misc/ or thirdparty/, so the hiding the editor code that
deep was no longer meaningful.
(Manual redo of 49c065d29c)