Bugfix for T69413

Fixes the invisible scrollbar when opening preferences by 'Editor Type' by always creating scrollbars for V2D_COMMONVIEW_PANELS_UI objects.

This removes a fix (T47047) for dpi issues when dragging out the toolbox. The new toolbox in 2.8 does not seem to have the same DPI issues.

Exact steps for others to reproduce scrollbar invisible (T69413):
change the 3d viewport to preferences and go to addons. The scrollbar will not be visible

To verify there are no DPI issues:
Apply the patch and open blender. Hide the toolbar. Split the 3d view port into 4 windows, such that one is very small horizontally, another is very small vertically, and a third is small in both directions. Drag out the toolbar arrow on all four viewports. The Toolbar will be the same size in each. For further verification, adjust the scaling and repeat.
{F7805968}

Also this is my first time submitting a patch. Some instruction is welcome!

Reviewed By: #user_interface, Severin

Maniphest Tasks: T69413

Differential Revision: https://developer.blender.org/D6050