Check GtkIMContext:input-purpose to disable IM on lock screen

Recent GtkIMContext has "input-purpose" property, which can be used to check if the target widget is a password entry, in a more reliable way.

BUG=none
R=Shawn.P.Huang@gmail.com

Review URL: https://codereview.appspot.com/7064059
Defalut triggers key is changed to <Super>space.

We have been used <Control>space as the default triggers key.
GNOME3 and Windows7 applied <Super>space as the default trigger key
and IBus follows it for the consistent key between desktops.

BUG=RH#953404

Review URL: https://codereview.appspot.com/9081043
Add man files of /usr/bin files.

Review URL: https://codereview.appspot.com/9413043
Let users know the default hotkey is Super+space with libnotify.

Review URL: https://codereview.appspot.com/9407043