callbacks: Refactor on_reload_as_activate() as document_reload_prompt()

The callback wasn't used from glade but is useful for some other places so
it's worth moving into document.c. This also fixes a bug where reload via sidebar
always reloads the current document instead of the actually clicked one.
Avoid passing non-literals to string formatting functions

Having a string literal allows the compiler to check the arguments
match the format, so avoid passing non-literal whenever possible.
VTE: avoid exporting callbacks that are only used internally
Add printf attribute to document_show_message() for better safety

Add G_GNUC_PRINTF attribute to document_show_message() so some
compilers can check the format arguments match the format string.