gconfig: add support to show hidden options that have prompts

There's a button in gconfig to "Show all options", but I think
normally we are not interested in those configs which have no
prompt and thus can't be changed, so here I add a new button to
show hidden options which have prompts.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>