Policy Plus - Fix list element REG export

I received a Policy Plus issue report on GitHub stating that the Export REG feature failed. After experimenting with the GPOs on a couple machines, I found what was necessary for the crash: list policy elements that clear the list key before adding values. When the PolFile representing the GPO tried to apply the key clearance, it requested the list of existing values in the target RegFile, which crashed because the REG file implementation of PolicySource is write-only. Changing ApplyDifference to use the ClearKey interface method instead fixed the problem.

But in the course of hunting down the issue and testing the fix, I found multiple other problems related to list elements. The POL editor window had an off-by-one error that caused a crash if adding a "clear the key before adding values" directive to the last key in the list - an easy fix. The list editor window (launched from the setting editor dialog) showed the **delvals. pseudo-value on subsequent launches of the setting editor for list elements that clear the key first. That was caused by the PolFile implementation of GetValueNames returning raw POL value names, which is important for the POL editor window but problematic here. For consistency with the RegistryPolicyProxy implementation, I changed the interface method implementation to return only true values and added another method to get raw POL entries.

The changes are live on GitHub.