Send "Disable" D-Bus signal to engine when it is unset.

Send "Disable" D-Bus signal to engine when it is unset.

As IBus does not destroy engines anymore, there is no way to allow an
engine to detect the input context is taken over by another engine.
This patch emits "Disable" signal so that the old engine can do proper
cleanup (e.g. hiding dialog).

BUG=none
TEST=manually

[Daiki Ueno, 2011-06-22 02:44:13]

I need this for ibus-gucharmap to hide charmap dialog when user switched to
another engine:
https://github.com/ueno/ibus-gucharmap/

Maybe this is not the right approach, but better than nothing.  Shouldn't
engines be destroyed on unset when global-engine is off?

[Peng, 2011-06-23 03:25:33]

On 2011/06/22 02:44:13, Daiki Ueno wrote:
> I need this for ibus-gucharmap to hide charmap dialog when user switched to
> another engine:
> https://github.com/ueno/ibus-gucharmap/
> 
> Maybe this is not the right approach, but better than nothing.  Shouldn't
> engines be destroyed on unset when global-engine is off?

I think the engine should be destroyed when it is not used anymore.

[Daiki Ueno, 2011-06-27 08:44:07]

On 2011/06/23 03:25:33, Peng wrote: 
> I think the engine should be destroyed when it is not used anymore.

OK, moving to https://codereview.appspot.com/4662043/