GQueue: accept a NULL sibling for insert_before() and insert_after()

It simplifies a little bit some code that inserts data relative to a
GList location, that might be NULL for the tail of the queue. A NULL
sibling is probably less useful for insert_after(), so it's more for
consistency with insert_before().

https://bugzilla.gnome.org/show_bug.cgi?id=736620
Simplify code that uses g_queue_insert_before() and insert_after()

g_queue_insert_before() and g_queue_insert_after() now accept a NULL
sibling.

https://bugzilla.gnome.org/show_bug.cgi?id=736620
docs: Update GParamFlags docs.

Remove reference to G_PARAM_READWRITE from GParamFlags' description,
since commit 6c395244a5db added G_PARAM_READWRITE to GParamFlags.

https://bugzilla.gnome.org/show_bug.cgi?id=726037
docs: Add missing opening parenthesis

https://mail.gnome.org/archives/gnome-web-list/2014-November/msg00003.html
gapplication: enable --help when app has options

This should already work according to the documentation, but doesn't
because main_options is consumed before the check in
g_application_parse_command_line().

Fix by moving the check for main_options up.

https://bugzilla.gnome.org/show_bug.cgi?id=740157
Updated Norwegian bokmål translation.