Add a method to create a Pixbuf with the preview in the metadata

This code was moved from sugar expandedentry.py to be available to use
in other places. Some activities like Portfolio or JournalShare
have this code copied.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Add a option to the objectchooser to show the object preview

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Merge branch 'objectchooser_preview-try2' of https://github.com/godiard/sugar-toolkit-gtk3 into godiard-objectchooser_preview-try2
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Merge branch 'master' of https://github.com/sugarlabs/sugar-toolkit-gtk3