ibusManager: Disable IBus input sources on password entries

ibusManager: Disable IBus input sources on password entries

Input method preedit text needs to be disabled on password entries
for security and usability reasons.
IBus 1.5.7 provides the signal set-content-type so that panel UIs can
handle these special purpose input entries:
https://github.com/ibus/ibus/commit/6ca5ddb302c9

Unfortunately IBus versions older than 1.5.10 have a bug which causes
spurious set-content-type emissions when switching input focus that
temporarily lose purpose and hints defeating its intended semantics
and confusing users. We thus don't use it in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=730628