Properly clean cache which binds inset and dialog.

Properly clean cache which binds inset and dialog.

We fill up edited insets into cache when editing inset is triggered, but
this cache is never cleared up for dialogs unassociated with some inset
- thus when e.g. graphics dialog is open for completely new image the
old cache is (wrongly) used.

https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg207192.html