Comment 20 for bug 23369

Hi, I want to give more comments on the patch, hope it helpful for reviewers
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This patch used some GConf APIs, no other Gnome APIs used.
However, the gconf lib is load manually, in order to build/run successfully
whether there is gconf lib or not.

The files in extensions/pref/system-pref/ is assumed to work for all desktops,
only extensions/pref/system-pref/gconf is for gnome (which uses gconf).

The system-pref module listens to the NS_PREFSERVICE_READ_TOPIC_ID just like
extensions/pref/autoconf does, and merge system-prefs into mozilla.

The system-pref is read-only for mozilla and has higher priority than prefs in
mozilla profile. This feature is useful for user/administrator to configure
applications system wide. This feature is disabled by default, users need it can
enable it by add the following line in his pref.js:
  user_pref("config.system-pref", true);

When this feature is enabled, and user want to change the prefs that is read
from system, the only method is to use the system provided pref editor, such as
gconf-editor in gnome.

Currently only two prefs are read from system-pref module, they are used for
examples, other prefs can be added freely. The tow prefs used in the patch are:
"network.proxy.http" and "network.proxy.http_port"
(see system-pref/src/sys_pref_list.inc)
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