This allows marking properties as "explicit". Properties marked like
this are included in the introspection, but are avoided in GetAll()
property queries, PropertiesChanged() signals and in in GetManaged()
object manager calls and InterfacesAdded() signals.
Expensive properties may be marked that way, and they will be
retrievable when explicitly being requested, but never in "blanket"
all-property queries and signals.
This flag may be combined with the flags for "const" and
"emit-validation" properties, but not with "emit-validation", as that
is only useful for properties whose value shall be sent in "blanket"
all-property signals.
The "explicit" flag is also exposed in the introspection data via a new
annotation.
Some places invoked fflush() directly with their own manual error
checking, let's unify all that by using fflush_and_check().
This also unifies the general error paths of fflush()+rename() file
writers.
This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.
