Fix Derma control bases and type ControlPanel:AddControl results

[Pollux12]

Summary

• Derive DTextEntry from TextEntry, and DCheckBoxLabel, DFileBrowser and DNumPad from DPanel, matching their real bases so inherited methods resolve.
• Type ControlPanel:AddControl results by control name (textbox, checkbox, slider, numpad, color, propselect, matselect, ropematerial, listbox, combobox and more), instead of returning a plain panel.
• Declare the panel classes those controls return that were missing: RopeMaterial (a MatSelect), CtrlNumPad and CtrlColor, with their fields and methods.
• Let skins add their own named colours to SKIN.Colours, read with derma.GetColor or SKIN.Colours.
• Let ents.Create return NULL for known engine entity classes too, since creation can still fail.

Confidence Score: 5/5

This PR appears safe to merge; no actionable defects were established.

Summary

This PR improves the types exposed for Derma controls.

• Control panel names now point to their specific control types.
• Four Derma controls now name their actual base classes.
• Skins can now declare extra named colours.
• Known entity classes can now return NULL in the type notes.

No actionable issues were established.

_{Reviews (1) · Last reviewed commit: "Declare the panel classes ControlPanel:A..." · Reviewed by Greptile}