Move the code used to get the Pixbuf from the preview to the toolkit

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Add a option to show the preview from the objects in the journal

Use a IconView and the preview images are generated as needed.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Clear query button should respect what_filter parameter

When he ObjectChooser was opened filtering by a file type (e.g. Images),
and the user filter something by text, and the query is empty,
when the user press the button to clear the query,
the initial filter should be respected.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Merge branch 'objectchooser_preview-try3' of https://github.com/godiard/sugar into godiard-objectchooser_preview-try3
Merge branch 'master' of https://github.com/sugarlabs/sugar