IPropertySheet: make it a standard C++ class with pure virtual functions (PR #892)

* Interface.h: ensure that functions in the interface are not exported by other header files.

This is generally harmless, except for the presence of default arguments, in which case the version with default arguments must come first.
To avoid the issue, these functions are only ever exported in the Interface.h header file.

source/Memory.cpp

@@ -1572,7 +1572,7 @@ void MemInitializeROM(void)
 		default:
 			{
 				_tcscpy(sRomFileName, TEXT("Unknown type!"));
-				sg_PropertySheet.ConfigSaveApple2Type(A2TYPE_APPLE2EENHANCED);
+				GetPropertySheet().ConfigSaveApple2Type(A2TYPE_APPLE2EENHANCED);
 			}
 		}
 
@@ -1674,7 +1674,7 @@ void MemInitializeCustomF8ROM(void)
 		}
 	}
 
-	if (sg_PropertySheet.GetTheFreezesF8Rom() && IS_APPLE2)
+	if (GetPropertySheet().GetTheFreezesF8Rom() && IS_APPLE2)
 	{
 		HGLOBAL hResData = NULL;
 		BYTE* pData = NULL;