Datastore: Add constants to the width and height from preview image

[godiard]

In the code there are many places where is generated a image for the preview, in all there are numbers for the width and height.

sugar/extensions/globalkey/screenshot.py:          preview = screenshot.scale_simple(style.zoom(300), style.zoom(225),
sugar/src/jarabe/desktop/favoriteslayout.py:       if d < 225:
sugar-presence-service/config.log:configure:2258:  result: /usr/bin/python
sugar-toolkit/src/sugar/datastore/datastore.py:    'preview': ByteArray(png file data, 300x225 px)
sugar-toolkit/src/sugar/activity/activity.py:      binary content of a png image with a width of 300 and a height of 225
sugar-toolkit/src/sugar/activity/activity.py:      pixbuf = pixbuf.scale_simple(style.zoom(300), style.zoom(225),
write/AbiWordActivity.py:                          pixbuf = pixbuf.scale_simple(style.zoom(300), style.zoom(225),
browse/downloadmanager.py:                         preview_width, preview_height = style.zoom(300), style.zoom(225)

We must add constants and change the code where must be used.
I can't do now because 0.90 it's freezed.

[godiard]

sugar-toolkit-gtk3 on sugar 0.100 provide activity.PREVIEW_SIZE to get the preview width and height

[dnarvaez]

Milestone 0.92 deleted