Avoid executing the query two times at ObjectChooser startup

The object chooser was setting at initialization properties
in the MainToolbox. Every set triggered a query change,
setting both parameters simultaneously, the query is executed
only one time. This reduce ObjectChooser startup time,
specially with big Journals.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Merge branch 'objectchooser-startuptime' of https://github.com/godiard/sugar into gonzalo-objectchooser-startuptime