[Deskbar] Non-threaded async handler

[Mikkel Kamstrup Erlandsen <kamstrup daimi au dk>]

The AsyncHandler I submitted yesterday had it's own thread to run
query() in. As I discussed with Raphael yesterday on IRC some apps/libs
are async by nature (libebook, gvfs and beagle fx) and manage their own
thread or thread pools.

This means that they can't use Handler and AsyncHandler will be overkill
(another thread atop of the one (s) in the app) - since a signal or
callback would suffice.

For this purpose Raphael suggested we have three handlers (to which I
agree)
1) Handler
2) AsyncHandler
3) SignallingHandler

-- or come up with a better name for 3) yourself :) Raphael suggested
GLibLoopAsyncHandler - I just figured that SignallingHandler was more to
the point... but anyway, no big deal.

I'll submit a patch later today (for 3), which hopefully will meat our
needs.

Cheers
Mikkel