Adding missing destroy call for Layer dialog boxes (Layer>Add layer and Layer>Rename)

Adding missing destroy call for Layer dialog boxes, for issue #5973 (closed).

When the _close method was removed _apply from layer-properties.cpp in commit 8184cd8d, it wasn't replace with anything. In other areas/code, this was replaced with destroy() . As a result, the dialogs for Add and Rename would remain open after clicking the 'apply' buttons.