dtwm: Cleanup some implicit definitions

Mostly this is adding appropriate #includes and declarations,
but for WmImage.c we also change from using the proper name
for XmeGetMask, rather than the identical but renamed
version _DtGetMask which is not exported in any header.

cde/programs/dtwm/WmFP.c

@@ -67,6 +67,7 @@
 #include "UI.h"
 
 #include "WmPanelP.h"
+#include "WmManage.h"
 
 #include <pwd.h>
 #include <fcntl.h>