Allow Plugin:StartDrag() to permit GUI parameter

The Plugin:StartDrag() functionality is very useful to plugin developers, but for the users, it can be hard to visualise what they are dragging, especially when what they are dragging is based on context.

For example, the naming of a customisable setting may change during the studio’s run-time, however, the current functionality does not allow us to see these changes when using the drag tool.

On top of this, the functionality, at the bare minimum, should allow the cursor to remain visible, on top of the dragged GUI. This change is especially necessary for PluginGUI’s because the content either become clipped or is difficult to distinguish.