Do not show property panel by default.

Property panel sometimes hides text entry box on firefox in case
the mode is auto-hide because firefox does not provide the right
cursor locations and firefox does not update cursor locales
when preedit is updated so property panel hides text entry box
for a while and auto-hide is not useful with firefox and thunderbird.

TEST=gsettings reset org.freedesktop.ibus.panel show

Review URL: https://codereview.appspot.com/90480043