Add a new command, "dialog-new", allowing external code (e.g. Nasal

Add a new command, "dialog-new", allowing external code (e.g. Nasal
scripts) to create dialogs at runtime.  Augment "dialog-close" to take
a name argument, allowing code other than PUI callbacks to close
dialogs.

The changes to the GUI directory to enable this are actually minor,
basically amounting to using SGPropertyNode_ptr reference counting
(the GUI subsystem no longer "controls" the dialog property trees, so
it can't delete them).