Add Property Panel to show IME properties.

Currently GtkStatusIcon can show one icon only while GtkApplet shows
multiple icons. IBus GTK Panel icon shows the IBus engine icon
however each IBus engine provides the properties and currently it can
be show only when users click the panel icon and show the activate menu.
Users wish to know the some of the properties without clicking the icon.

Property Panel offers a window which shows engine properties' icon
at input cursor. It's similar to Language Panel in IBus 1.4.

Property Panel is shown in only 3 seconds whenever client applications
get focus-in event or any of engine properties are changed,
e.g.  InputMode is changed from Hiragana to Eisu,
in case gsettings panel.show is AUTO_HIDE (by default).
Property Panel is always hidden in case panel.show is DO_NOT_SHOW.
Property Panel is always shown in case panel.show is ALWAYS.

BUG=http://code.google.com/p/ibus/issues/detail?id=1659
TEST=ui/gtk3/ibus-ui-gtk3

Review URL: https://codereview.appspot.com/40870046