Avoid warnings when opening popup or hiding/restoring decoration.
With the present GTK popup, warnings occur when 1) hiding/restoring the decoration (type key 'h' twice) and when 2) showing/hiding a popup window (right click).
The proposed MR removes all of these warnings by adding several if (GTK_IS_WIDGET(pointer)) tests and by implementing the seat_name(void *) function just as libdecor-cairo.c does.
The MR also sets function libdecor_plugin_new() static rather than exported just as libdecor-cairo.c and libdecor-dummy.c do.
Edited by ManoloFLTK