core: Avoid pedantic compiler warnings with C++.

g++ complains about trailing commas in enums when using
-Wpedantic.

This fixes https://bugzilla.gnome.org/show_bug.cgi?id=679674

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
doc: Clearify that GrlConfig are not persistant
doc: Fix typos in GrlConfig description
grl-net: avoid a possible double free

In the soup-unstable there was the possiblity to face a double
free if get_content() was called twice with the content parameter
set as NULL first.

Thanks to Jens Georg for aiming at this issue.
core: Fix typo in key description
vala: Do not install VAPI files in specific vapi directory of the compiler

Rather, install it in $(datadir)/vala/vapi place.
vala: make container in source.browse() nullable
core: Fix typos in documentation
doc: Improve documentation
Update Copyright headers
Bump to 0.1.20