Remove prototypes of non-existing callbacks
Don't export callbacks not used from Glade
Normalize callbacks prototypes
Move Glade-only callbacks out of callbacks.h

There is no need to expose the Glade-only callbacks in the header as
nobody uses them directly anyway;  and not having them makes the header
a little less bloated.
Remove redundant toolbar-specific callbacks
Add two missing Glade-only prototypes
Use the appropriate prototype for the window's delete-event callback
Don't manually connect handlers Glade can connect for us

Don't bother manually connecting handlers that Glade can connect for us
just as well.
Move on_clone1_activate() to callbacks.c
Callbacks: get rid of the ugly `insert_callback_from_menu` global
Move include insert callbacks to the UI code where they belong
Move date insert callbacks to the UI code where they belong
Remove line number zoom hack as it is no longer necessary

Since cbf9813632d7ee921ffa1fda0d1f69128b2bc755 we have a proper
solution for maintaining line number consistent with the editor zoom,
so drop the old hack.

This also allows to remove the now useless third argument of
sci_set_line_numbers().
Merge branch 'callbacks-cleanup'