Allow Middle Click + Drag to scroll in docks

[mich-gamedev]

If you hover a scrollable area in webpages or apps, it will often let you drag the mouse while holding middle click to scroll it, however the functionality is missing in Godot.
Since Godot also uses middle clicking to pan the view-port, it can add more consistency within the editor and with other apps, and would be more accessible for the numb-handed.