Allow optional manual ordering in the Solution Navigator

Keep the Solution Navigator's current alphabetical arrangement by default, while offering an optional Custom order for people organising large solutions. In Custom order, source items and virtual folders can be placed in a chosen order within the same folder. The chosen view is restored when the solution is reopened on the same computer.

Reordering must be clearly distinct from moving an item into another folder, project, Shared code, Tests, or a module. Existing moves continue to work as before.