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).
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
ActivityFactory: don't run which to check for rainbow

One less thing to do in the common activity startup path.
Also, which might not be available.
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Release 0.99.1
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Merge branch 'master' of github.com:sugarlabs/sugar-toolkit-gtk3
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user mschlager.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).
Commit from Sugar Labs: Translation System by user cjl.: 40 of 40 messages translated (0 fuzzy).